depends...with a stock chip, low altitude and 92 octane, it should handle 15psi to start with. I'm at 4200' altitude, have a chip with more timing, and we have no better than 91 octane gas around here. When I finally got my boost gauge, I found I was only running 8psi! So, I cranked in the rod 4 turns, which gave me 12, and BIG difference! So I went two more turns and hit 14psi, still okay...
Right after that, the wife needed to go someplace, so I went for 16psi. About 10 minutes into our ride, I had some room for WOT for a few seconds. I watched the gauge, and the second it hit 16 I got a single, extremely loud <WOCK>...
It sounded like a cannonball hit the block! And the boost gauge dropped to 10psi before I could back off the throttle! (wastegate solenoid doing it's job!)
I backed it off to about 15, and it's been there ever since. I can't hear it knock, but I know that it is, simply because I can feel the car surge when the timing is being backed off by the ECM, and the boost will back off to 14...
Anyway, I think 15psi is a good starting point!