Basically, any shorter than it is, and you'll start having to use gas setting 2 almost exclusively. How much shorter before setting 2 won't run what you want is going to be VERY individual gun dependent.
Really depends on how well the gas ports were aligned when your gun was assembled.
I got lucky with one of mine, it will run everything on setting one. I've shot others that have to be on 2 for anything under 1 oz or 3 drams.