I've got a clinometer on mine. I've had it to 40 degrees when messing around
trying to answer that very question. I may have had it further, but once
you're over that far you're no longer paying much attention to the
clinometer.
Performance isn't so good heeled that far, but the boat comes back up (or
broaches, then comes back up!). I am of the belief that a potter sails best
on her feet, so I try to keep the heel below 15 degrees or so.