Great question, and the simple fact is, there is no commandment to "tithe" in the way we do today. The 10% which is often suggested (or even required) by some churches today is a misapplication of the scriptural tithes of Leviticus, Numbers, and Deuteronomy.
The first time was see any mention of a "tenth" is in Genesis 14:18-20, when Abraham gave Melchizedek a tenth of everything he has obtained from battle. The first time we see a "tenth" offered to YHWH, is in Gen 28:20-22, when Ya'akov states he will return a tenth of everything to YHWH.
In the books of Leviticus, Numbers, and Deuteronomy, we find a complex system of tithing a tenth tied to the year of the Sch'mitah (the seventh year forgiving of debts). There is a tithe every year, every 3rd year, and every 7th year (the year of the Sch'mitah). Suffice it to say that a tenth is given to the Levi who manage its use. Indeed, since the Levi had no part in the inheritance of the land, YHWH made it clear that the tithe was in part to support them. The tithe also supported foreigners, widows and the poor.
It is this form of tithing that was commanded, and thus still exists today - though with no temple and no Levi in charge of the Temple, it's pretty hard to comply with the tithing commandments.
By the way, it is fallacy to argue that the "tenth" spoken of in the Tanahk ("Old Testament") is only agricultural. Hebrews 7 speaks of the tithing of "income": Just think how great he was! Even the Patriarch Avraham gave him a tenth of the choicest spoils. Now the descendants of Levi who became cohanim have a commandment in the Torah to take a tenth of the income of the people, that is, from their own brothers, despite the fact that they too are descended from Avraham." (Hebrews 7:4-5)
So what is our obligation to tithe today? It is in the act of giving.
Proverbs 2: 9 He who is generous is blessed, because he shares his food with the poor.
Since we are no longer an agricultural society, then taking what we use today as our medium of exchange (money) if we give of our money to help the poor, then we are in obedience with Proverbs 2:9.
We also have 2 Corinthians 9:9 which says "as the Tanakh says, 'He gave generously to the poor; his tzedakah lasts forever.'" This is a reference to Proverbs 2:9!
Psalm 37: 21 The wicked borrows and doesn't repay, but the righteous is generous and gives.
Isaiah 32: 8...the generous person devises generous things, and his generosity will keep him standing.
1 Timothy 6:17-18: As for those who do have riches in this present world, charge them not to be proud and not to let their hopes rest on the uncertainties of riches but to rest their hopes on God, who richly provides us with all things for our enjoyment. Charge them to do good, to be rich in good deeds, to be generous and ready to share.
So are you "tithing" when you give? Must it be "10%"? You decide. Until the Temple is rebuilt, and the Cohen are put back to work, there is no "tithe" as the Church would have you think, but count your blessings, and give till you have helped. In that way you have "tithed".
The types of Tithe:
* Yearly Tithe: "Every year you must take one tenth of everything your seed produces in the field" (Deut 14:22). In this tithe, a tenth of everything - be that grain, produce, or cash income - is taken to the Temple. The scripture goes on that you "eat it there" at the temple, but what this means is that upon gifting it to the temple, all who share in it eat it there at the temple after the Cohen (priest) has accepted it, blessed it and prepared it. It does not mean the giver has to eat all his own tithe. In this tithe, if it is too far for you to travel to the temple, you can sell your produce, seed, or animals for cash and take the cash to the temple and then exchange the cash for what you will consume at the temple.
* 3rd Year Tithe: "At the end of every three years you are to take all the tenths of your produce from that year and store it in your towns" (Deuteronomy 14:28, and Deuteronomy 26:12-14). This tithe is not delivered to the temple. It is stored in towns - again held and administered by the Levi - but it is for the Levi, foreigners, orphans and widows in the towns. (This is apparently not an additional tithe to the yearly tithe; rather at the end of the 3rd year what remains of that year's tithe is taken to the towns for storage and distribution.) With this 3rd year distribution, there was clearly plenty for distribution to the needy for the 3 years between tithes.
* 7th year Sh'mittah: As part of the other commandments in the 7th year, the Sh'mittah, when debts are forgiven and claims are released, there is a command to provide for the needy.
Deuteronomy 15:7-11: "If someone among you is needy, one of your brothers, in any of your towns in your land which YHWH your God is giving you, you are not to harden your heart or shut your hand from giving to your needy brother. No, you must open your hand to him and lend him enough to meet his need and enable him to obtain what he wants. Guard yourself against allowing your heart to entertain the mean-spirited thought that because the seventh year, the year of sh'mittah is at hand, you would be stingy toward your needy brother and not give him anything; for then he may cry out to YHWH against you, and it will be your sin. Rather, you must give to him; and you are not to be grudging when you give to him. If you do this, YHWH your God will bless you in all your work, in everything you undertake - for there will always be poor people in the land. That is why I am giving you this order, 'You must open your hand to your poor and needy brother in your land.'"
Note there is no specific amount commanded (i.e., no "tenth" is specified; just give generously).
* Levi Tithe: Numbers 18: 26 "Tell the L'vi'im, 'When you take from the people of Isra'el the tenth of the produce which I have given you from them as your inheritance, you are to set aside from it a gift for YHWH, one tenth of the tenth." So this is a tithe from the tithe! But this tithe is FOR the Levi, for his payment for his services in the temple, and the Levi can take it home to his family and eat it anywhere. (Numbers 18:31).
Now to the issue of whether or not "tithing" is a "forever" command: The short answer is that it is forever! But since there is no temple, and there are no Levi to carry out their duties in the temple, the "forever" requirement is simply "on hold." The commandment still exists; we simply can't comply exactly as YHWH intended!
The justification for tithing still being mandatory is this: When YHWH is explaining to Aharon his and his descendants responsibilities as Levi, He tells Aharon that their responsibility is a perpetual covenant. In this perpetual covenant is the handling of all the tithes! "YHWH said to Aharon, 'I myself have put you in charge of the contributions given to me. Everything consecrated by the people of Isra'el I have given and set aside for you and your sons; this is a perpetual law.'" (Numbers 18:8)
And: Numbers 18:19-24: "All the contributions of holy things which the people of Isra'el offer to YHWH I have given to you, your sons and your daughters with you; this is a perpetual law, an eternal covenant of salt before YHWH for you and your descendants with you." YHWH said to Aharon, "You are not to have any inheritance or portion in their land; I am your portion and inheritance among the people of Isra'el. "To the descendants of Levi I have given the entire tenth of the produce collected in Isra'el. It is their inheritance in payment for the service they render in the tent of meeting. From now on, the people of Isra'el are not to approach the tent of meeting, so that they will not bear the consequences of their sin and die. Only the L'vi'im are to perform the service in the tent of meeting, and they will be responsible for whatever they do wrong. This is to be a permanent regulation through all your generations. They are to have no inheritance among the people of Isra'el, because I have given to the L'vi'im as their inheritance the tenths of the produce which the people of Isra'el set aside as a gift for YHWH. This is why I have said to them that they are to have no inheritance among the people of Isra'el."
So, yes, tithing is a "forever" command; otherwise the Levi's perpetual covenant with YHWH could not be met.