If you are shooting under USPSA 3-gun rules, you are limited to 10+1 capacity in Open and 8+1 capacity in Limited. Elsewhere, who knows. As a point of reference, the Choate 10-shot mag extension projects just fractionally beyond the muzzle of a 26" barrel, so with a 30" you could get MAYBE 2 more rounds before it looked silly.
The longest commonly available mag tube is the Choate 10-rounder... if you want longer, you'll have to find someone to weld on some extra pipe (the unit on page 23 looks like it was made this way). In England, where practical shotgun is comparatively popular, some guy was custom making longer mag tubes, but I've no idea if he still is, nor does his contact info fall readily to hand.
ETA: Found the guy in the UK. Contact Tim Wander (
[email protected]) and see if he still makes the custom tubes. The original advert is on
this page.