It depends...
...on your keel configuration and on sea states. Probably somewhere around 17-18 degrees on most of these boats. If you find that you're going faster once you reef, then you were heeled too much. Don't just pay attention to your inclinometer, though, look at your wheel. The kingspoke should be marked so you can see how much weather helm you've got going. You want it to be somewhere right between a little and a lot. If the wheel is hard over, the boat is not going as fast as it could be. When that rudder gets sideways it becomes a brake.