About 2 hours if the traffic is good. Expect lots of traffic between Heidelberg and Heilbronn, especially Friday afternoon/evening. Add a couple hours if you chose to drive during that time-frame.

A81 is a nice "racetrack". Little traffic and fun curves.

Don't be suprised if you see a "prototype" car running on that stretch. I've seen the camoflauged Porsches (with the black vinyl on them), go ahead and have some fun with them.
