Obviously the more thorough the inspection the longer it will take. There are additional services if you want as well. Do you want a mold test, etc… For say a 2ksf home look at anywhere from 2.5 – 3 hours for a good thorough inspection. Phillipe Heller is a good guy at San Diego Home Inspection. Google him and you should be able to find him. He also has an infrared for moisture testing if you feel there are some suspicious areas in the home. Had some clients in escrow a few weeks ago and we were concerned about a portion of a ceiling under a rooftop deck. The listing agent said it was repaired and totally dry and the sellers said it was to. We found with the infrared there were 3 major moisture intrustions.
If you want a guy to come out and do a full moisture test of all the walls and such that is a bit more involved and probably not needed if the home is in decent condition.