There is a good article about this on twist rates on ar15.com. Here is a rough guidline:

1-in-14" 55gr or less
1-in-12" 35gr up to 55-60gr
1-in-9" 45gr-75gr, possibly up to 77 if you're lucky
1-in-8", 1-in-7" 45gr - 80+gr

I think 1/9 used to be a good general twist rate to shoot everything, but guys who shoot matches started to use 1/8 cause it stablizes the heavier bullet better.