I prime for 60 seconds EVERY time I start the generator, timed with my wristwatch. You can not over prime. The gas just cycles back into the fuel tank, or stops getting pulled from the fuel tank depending on how your fuel system is plumbed.
Even priming the engine, it takes me about 4 attempts to start my generator. I crank it for about 5 seconds, then stop, and repeat (the starting, not the priming).
Running your generator for 2 hours every month under at least 50% load (as Cummins/ONAN recommends) will prevent fuel varnishing in the generator. ONAN says it is better to do 2 hours once a month than 30 minutes a week.