Charlie,
Crating dogs for short periods of time is fairly common now, but I don't know that I've ever heard of a "specific" time limit - for example: "2 hrs max, 3 hrs max", etc.
I'd suggest contacting your vet's office - they may be able to give you some guidelines, OR they may be able to recommend another client/dog owner (or local dog groomer or reputable breeder) who is familiar with crate training.
In the meantime, check your private msg's - I sent some video clips & websites that may be helpful & answer some of your questions.
Terri, the Chevy co-pilot