Frankfurt Airport

I've left the airport from the farther out Fernbahnhof 5 times, and I've always been at the Bahnhof within an hour of arriving. However, that's an hour after arriving, and I've been an hour late arriving in Germany 30% of the time. If you are only "scheduling" an ICE, i.e. purchasing a fully flexible, full fare ticket or just a reservation to use with a rail pass, OK, but if you are buying a SparPreis (Savings Fare) ticket, no matter how much time cushion you leave, you could still miss the train.

It also depend on how far out on the concourse your gate is, or in which terminal. Non-Lufthansa or -Star Alliance airlines probably use Terminal 2, and it takes longer to get from there to Terminal 1 using the SkyLine, then to the Fernbahnhof. A couple of years ago, we arrived at one of the last gates in the "new" A concourse, and it seemed we walked forever just to get to the main hall of Terminal 1 (but we still were probably at the Fernbahnhof within an hour).

If you miss the train with a train-specific SparPreis ticket, you don't lose it entirely. You deduct a 17,50€ penalty and can use the balance of what you paid towards a full fare ticket, but I think it's a bit of a hassle to do it (I've never tried).

If you are thinking of building in a big time cushion in case the plane is late, depending on where you are going, it might be just as fast and inexpensive to just buy a regional ticket and use it as soon as you get there.

I totally agree with Lee. For me, I buy a walk-up ticket when I get to the train station after a flight. It's too easy for flights to be delayed to try and get a discount ticket on the same day as a flight.