I have read somewhere that an impeller will be damaged with as little as 15 seconds of running out of the water.
When I did my PB2 I was told by the instructor that the night before any trip I should start up the outboard to make sure all was well, then again before leaving in the morning, then again before launching before finally trying her after launching. All apart from the last "dry"
Needless to say I didn't ever follow his advice and on the course his outboard seized
You get some strange ideas from folk , while I don't believe a short try of the engine is going to completely destroy an impeller in seconds, doing it again and again or running it for a any length of time will and it is not something you really want to be doing if you can avoid it.