Well, I think 5 hours is about right.
Driving from Frankfurt to Amberg (A3 and A6) is fine (i.e. no speedlimit most of the time), but then you have two pieces where the Autobahn hasn't been completed yet to the Czech border (At least as of last summer, when I drove it last time). I haven't had any waiting times at the border, but you will have to stop to pay for the interstate pass (~10EUR for three days, I believe).

Also, in the Czech republic you have a speed limit (130, I believe) which is especially enforced if you are a foreigner (i.e. don't have a Czech license plate)

Additionally, don't be surprised if on the way back you are stopped after the border by German Customs ("Zoll"). Since a lot of people from the german side of the border drive to the czech side for shopping, there are quite frequent checks, especially on weekends.

David

Added: I agree with Aviatrix that I don't see much point driving this. If you want to take a car for a ride, you might just want to go to Nuernberg or Amberg - closer and you still get most of the "high-speed" Autobahn that you'd get if going to Prague...