My garage door opens over the car, so I often have fine debris falling on my car from the wooden ceiling/door - no air filters in my garage. I'd never dream of leaving my car uncovered so I can protect it from garage dings, dust, bird crap, insect crap, etc. I don't use the original car cover, but instead bought one from Calif Car Covers that is padded and keeps the dust off.
I don't have a time limit as to when to cover the car because it depends on the outside air temperature. I usually park the car and then just before going to bed, I cover it. I also feel above the lid vents with my hand and if I don't feel any heat escaping, I cover it.