Pertronix is very vague about how long it takes to damage the unit ... they claim it's from overheating the coil which should take quite a long time. So I would feel safe leaving the key on for 10 minutes or so, which should be plenty of time to complete static timing.
Also, although Pertronix doesn't mention it, if you do need longer to work with timing, I believe it would be safe to use a test lamp as a load in place of the coil. The test lamp will draw much less current through the Pertronix module, so it can't overheat.
But since I've not installed my Pertronix yet, these are only theories. YMMV