How long a flash is too long? Most high power monolights have flash durations of 1/600 second or shorter on max power. Using a lower power setting results in shorter times, with some lights that can be 1/2000 second.

If you want to photograph water drops and splashes that should work. If you want to "stop" bullets in flight or freeze exploding light bulbs I think you'll have to do a little construction.

Sherman