For smoothbore, 50 - 75 yards for most shotguns. My friend can shoot his accurately out to 100 yards, but from my experience, that is the exception rather than the rule.

With rifled shotguns, the only experience I've had is with the H&R single shot full rifled. That was extremely accurate with Lightfield slugs - like 2" groups at 100 yards. It is my cousin's deer gun, and he has taken deer out to 120 yards with it. I'm sure it would be good to 150+ if you practice to account for the bullet drop.