I clean my cameras once a week whilst cleaning the windows (quick wipe with a alcohol based wipe)... As for your domes you shouldn't need to clean the inside of the dome after installing unless you have moisture build up. If that is the case I would be looking at my water tight seals and check you have a silicon gem packet inside them.
Domes can be more susceptible to dust / water marks if located without any covering (fascia / soffits) so I guess you'd no better how dirty they are and how often you need to clean them.