Usually around a 60-65hp shot. That's a good safe range. You can go as high as 70-75ish, but that's really pushing it on a stock engine.
I built my engine to run on a VERY healthy shot of nitrous if I want to. Main thing is ring gap. Your stock engine is made for good emissions, a very tight seal on the pistons, meaning a very tight ring gap. Nitrous makes the engine run hotter, so does running a smaller supercharger pulley. More heat causes parts to expand more than they were designed to, and the rings are the main cause of peoples chipped pistons. The rings will expand till they cant go anywhere, and the weakest part is the top of the piston, and pop goes the top.
That's one thing I did on my build was ran a larger ring gap because of the cam, pulley, and random times nitrous use.
Pics. Page 5 of this thread should have some:
http://www.grandprixforums.net/belt-slip-thing-past-now-people-5-6354.html
~F~