When? Workday or weekend, nighttime or daytime?
The Autobahn from Nürnberg to Walldorf (A6) is under heavy construction right now and always busy between 6am and 8pm from Monday till Friday. From Walldorf you continue on the A5, busy during the rushhours, especially in the Karlsruhe area.

The distance is aprox. 330km, which can take up to 5 1/2 hrs on a Friday starting at 3pm, or can be made in 2 1/2 hrs (or faster, depends on you're driving) starting on a Saturday or Sunday at 6 pm. If you can't go on a weekend, try to start after 7pm.