It doesn't really go bad, but the stuff can settle out of solution. You'll want to open up the container from the back and stir the stuff up with a clean metal object before using it.
Or just get new TIM. I like AS5, but with my most recent CPU install I used MassCool Shin Etsu, and the stuff is working as well as AS5 does, without needing time to cure. I've also heard good things about OCZ Freeze, IC7, and MX-3.