According to the source code, level 20. I'm not sure whether or not this is the actual level, or some other number, though (but it said that max level is 50, so I'm pretty sure it's the actual level).
Check the wiki. Don't know about the minimum, but I just did a lvl48 enchant on a bow and got power 5, punch 2, and infinity! This was on hardcore too, so it will come in handy for sure!
Don't Ask
If you look at the link below, optimal is between 21-24 XP levels spent.
http://pernsteiner.o...Infinity_1.html
by c0yote
I tried it with terrible results. I gave my wife my glasses for a second, a creeper showed up and now my wife is pregnant.
Stupid 3D..
Well that was one necrobump. You could start a new Thread you know. Also, max level 30 was implemented in 1.3.