This article is from the Stand-up Comedy FAQ, by Steven Rosenthal and Steve Silberberg (firstname.lastname@example.org) with numerous contributions by others.
A. Tapes should be from 5 to 15 minutes. From different shows, if possible.
Club owners that have to see a 30 minute tape are more concerned with
quantity than quality. YOU know whether you can do 30 minutes or not. The
owner of Guffaw's Pizza 'n' Comedy Castle may have a different perspective,
but too many clubs have comic's tapes as doorstops, coasters, etc. And see
if you know a headliner that works the club regularly. Maybe they can
recommend you OR bring you in as their middle. Better not suck, though, or
you'll lose a booking and a friend.