A modern fuel injected gas engine can idle for long periods and be OK, but beware there is no 'safe' distance to be sleeping near a running engine.
The main issue with idling both gas and diesel is condensation of fuel on the cylinder walls. It mixes/dilutes oil causing other problems.
Big rigs are idled for 10+ hours at at time...especially in cold weather where they are afraid the engine might not start in the cold. A buildup of fuel oil will form in the exhaust system called slobber. Here is a trade story about how to correct it in diesels...should apply somewhat to gas engines w.r.t possible plug fowling. Gas engines will not show slobber like a diesel. The Catalytic converter will burn it up.
http://findarticles.com/p/articles/mi_m ... _113053279
If it were me, I'd idle my engine up to 1 hour before revving it up. I'd also never sleep with the engine running or my microlite generator(without an exhaust stack)