We arrive in London next week, and then have a flight to Frankfurt. British Air told me that we wouldn't go through customs in London, that we would in Frankfurt and that it could take up to 1 1/2 hours. We were hoping to get out of there fast to catch the train to Rothenburg where we have reservations for the night. Is that realistic? We would have thought that the customs would have taken place in London since we have a 3 hr lay over.
The UK isn't part of the Schengen Zone, so immigration and customs will be in Frankfurt. 90 minutes is more of a worst case scenario, it all depends on how many flight arrive at the same time; it could take as little as 30 minutes.
Most likely it won't be customs that takes so long, it is passport control (immigration). It does indeed depend on how many flights are coming in at the same time, so there is no way to guess. Customs, which concerns things you might be bringing with you, is just a walk through the green "nothing to declare" door, unless you are selected for further scrutiny (unlikely).
The immigration line is where you'll have the longest wait. Customs is a walk-through. I don't think I've ever waited longer than 15 minutes for the immigration line in Frankfurt. Also, if you check your luggage, baggage retrieval at Frankfurt is a little slow. Add anywhere from 15-30 minutes after your flight lands. Of course, you'll wait in the immigration queue first, so that might cut down the amount of time waiting for your bags. After you pick up your baggage, then the next step is customs, but as previously mentioned, you merely walk through a set of doors without stopping. It will take you another 10-15 minutes to walk to the airport Fernbahnhof. BTW, I'm sure you're aware already, but just in case... there are no fast ICE trains to Rothenburg odT. From the airport, you would need to take an ICE probably to Würzburg first, then switch to a slower regional train to Rothenburg... and I'll save my usual Rothenburg rant for another time...