Minimum PWM speed of a light to prevent flickering?
The quick answer: Drive them with DC current, and avoid time-sensitive behavior. 

If you must use PWM, see Spehro's comment.

The long answer: examine the time-sensitivity curve of the fastest film you'll ever use (or luminance response time of your camera's electronic sensor (CCD, CMOS) at its highest ISO setting). 

Electronic sensors can be very fast. Dalsa has a video camera that can shoot 100 million frames per second. I would use continuous-power lights for that application!