Really really depends on what you want to do.
I started a band when I'd been playing about a year, with some people who'd been playing about the same time. In the six months it went for we wrote a bunch of cool riffs but finished nothing. For the next year and a half or so I played by my self, writing material, and just recently formed a new band (with some of the same people), and it's going much better. So while I'd like to say that to start a band and sound good you don't need any experience, just the right mindset, I don't think that's true. What is true, though, is that the only way you get good is by practice and you get better a hell of a lot faster playing with other people. So start up a band and jam away. To join a band that plays reasonably complex material a guitarist doesn't need much music theory but will probably want to have been playing for a good 2.5-3 years.
The poster above me said start a band with people at the same level. This is a good idea as if someone is better they'll be bored. If they're worse than the others, though, encourage them to practice and they'll catch up fast.
Thrash metal is varied in terms of technicality. There's some stuff that requires really really fast alternate picking and precise syncopation, and there's some stuff you can play after maybe a week.