The Mil-Spec crowd will insist on a 1:7

That said, the most common is a 1:9.

1:12-1:14 will stabilize most bullets (35-63 grains) in a 223/5.56 with the exception of the heavy for caliber bullets like those VLD's made for very long range (69-90 grains) and the long for weight bullets like military tracer rounds.
1:9 will stabilize most bullets up to 75 grains (except tracers)
1:8 will stabilize almost everything (very few exceptions)
1:7 will stabilize them all but giving up velocity due to the higher spin and some lighter weight thin skin varmint bullets come apart before ever reaching target at distance. This has been well documented with v-max bullets.

With all that said I have no use for tracers so I don't own a 1:7. I do however own mostly 1:9's and a few 1:8's. For long range shooting for bullet weights up to 80 grains I have a custom built upper with a 1:10 which shoots sub MOA with 80 grain and under bullets from a 20" barrel.