You can totally go 250km/h on the A5 from Frankfurt to Darmstadt, if traffic allows it. That wont always be possible, but typically if it isnt during work traffic, 160-200km/h is doable comfortably, even on other parts of the Autobahn
The longest unrestricted part that ive driven should be the A71, where you can go longer passages without restrictions, save for the tunnels.