For suppressed use, I'd go as short as you can without giving up the effectiveness of the cartridge. A 20" or 22" 308 would be fine and not give up much velocity. I wouldn't go shorter than 18" though.
I borrowed my son-in-law's Kimber 84 .308 for this year since my rifle is waiting for a new barrel. Its barrel is 18" and gets 2,775 feet per second average for the 155 grain bullets. The powder is IMR3031.
