generally about 7" for 5.56/223 though 4th is working on a AR-15 derringer in 5.56--LOL!
no limitation for pistol rounds in blow-back.
the deal with pistol cals running DI is there about the same length as a pistol barrel and i am thinking shorter would be better on the gas block with them.
now here is an idea for super short 5.56 like a 5"-- --go with a gas piston set up!
even the cheap gas piston kit has an adjustable gas block that could come in very handy--