I blew a cheap cap apart at 35 PSI, ricocheted off my gloved hand (still bruised it) and broke my heater core line off. I highly recommend only running as much air pressure as you absolutely have to. If you plan to run the stock turbo, I'd only check for leaks at 25-30 PSI. In retrospect, I might have tightened the clamp too much, and stress cracked the plastic before I even started?
After that fiasco, I made one out of a spare chunk of aluminum, and I've gone 40+ PSI with no problem. I run more boost than that, but if it doesn't leak at 40 PSI, I figure it can't leak much at 50 PSI.
Edit: wear safety glasses, no matter how stupid you think it makes you look. You'll look even stupider picking plastic shards out of your eye. I was glad I was wearing them when my adapter exploded. Or maybe i just have really bad luck?