Dear Nealglen37:
Your own range work will answer your question better than any opinion. Most results, and opinions, suggest 50 yards or so, as a starting point, and sometime as a limit, as far as accuracy. Lots of barrels and chokes out there.
May I suggest the NEF shotgun forum, on this site, for info on range and ammo? Some of them boys are scarey accurate!
I'e seen the results, on deer, that a 20 ga does, but usable range is a bit limited with my particular gun. Woodbutcher