The word tithe literally means "tenth", so by definition, a tithe is a tenth. I believe that's why the teaching originated the way it did.
The idea of the tithe comes originally from Genesis 14, when Abraham gave Melchizidek a tenth of everything he had:
And Melchizedek king of Salem brought out bread and wine. (He was
priest of God Most High.) And he blessed him and said, “Blessed be
Abram by God Most High, Possessor of heaven and earth; and blessed
be God Most High, who has delivered your enemies into your hand!”
And Abram gave him a tenth of everything.
Now, today, there are many people who would say that a Christian should give 10 percent of everything they have, thoughts, money, time, talent, etc to the church.
Unfortunately, there are a couple of problems with this statement.
Firstly, nowhere in Scripture does God demand, and nowhere in Scripture is it demonstrated that one should give back 10% of anything other than money. Even the 10% given by Abraham was a voluntary 10% he chose to give because of the blessing given to him by Melchizedek. Thus, this viewpoint of things other than money is an extension of the law, and is not supported by Scripture. Is it good to give of your time and other things? Sure. Is it required by the Law? No.
Secondly, Paul, in the New Testament indicates that people ought to give as they see fit (2 Corinthians 9:7):
Each one must give as he has decided in his heart, not reluctantly or
under compulsion, for God loves a cheerful giver.
So here, Paul is saying that we ought to give as we have decided in our heart, not out of compulsion. In my opinion, to require somebody to give 10% is really demanding money out of compulsion. Most listener's response to hearing this would be something along the lines of, "I need to give 10% because my pastor has asked me to do so."
Lastly, and I think this is the most important part, when we set these kinds of laws before us, the chief danger isn't that we'll break the law, but that we won't break the law, and in not breaking the law, we begin to depend on our own goodness to earn God's favor. Depending on our own goodness and our own works is antithetical to the Gospel.
If you are in Christ, and if you believe that Christ's death pays the penalty for your sins, you're free, and not bound by any law any more. That includes the law of tithing. You're free. Give as you see fit, and don't obsess over the numbers.
This can be summed up with a couple of verses.
1 Peter 2:16:
Live as people who are free, not using your freedom as a cover-up for
evil, but living as servants of God.
For you were called to freedom, brothers. Only do not use your freedom
as an opportunity for the flesh, but through love serve one another.
When we see we're free, we'll give out of our gratitude for Christ's sacrifice, and not out of conformity to the law. And so, like the Macedonians in 2 Corinthians 8, we may actually end up giving more than the 10%, but it will not be because of the requirement set forth in the law, but rather because we've been set free from it.
2 Corinthians 8:1-5
We want you to know, brothers, about the grace of God that has been
given among the churches of Macedonia, for in a severe test of
affliction, their abundance of joy and their extreme poverty have
overflowed in a wealth of generosity on their part. For they gave
according to their means, as I can testify, and beyond their means, of
their own accord, begging us earnestly for the favor of taking part in
the relief of the saints—and this, not as we expected, but they gave
themselves first to the Lord and then by the will of God to us.
So to answer your question: No, we do not have to tithe 10% of our time as well.