Customs upon arrival in Frankfurt

Customs is one minute or less if you walk fast. However, immigration (passport control) could take longer depending on the number of flights at the same time and number of processing officers. BUT -- you should be OK if flight is on time and things are running as normal. The Germans are pretty efficient and Frankfurt is a decent airport to land at.

Standard comment: Check whether your airline offers Rail&Fly tickets, which are both flexible and cheap.

As Frank points out, passport control and customs are two different things. Passport control (or immigration) is where an agent looks at your passport, scans the code into a computer to check for reasons you should not enter Schengen, then probably stamps it. Everyone is checked. You go to Immigration immediately after landing, before getting your luggage. There are only a fixed number of booths in the room, and if a large number of people arrive at the same time, it might take a while.

Customs in theory is a check to see if you are bringing anything prohibited into the country or whether you owe a duty for commercial items you might be importing. It is usually a formality, a self declaration. There are two "lines" leading out of the baggage claim room. One says "nothing to declare". You usually just walk out of the room using that line.

Two and a half hours should be more than enough time to go through immigration and customs at FRA. I have only a carry-on so I don't have to wait at the luggage carousel; I have always arrived at the Fernbahnhof (when I use it) within an hour of landing. But I have arrived in Frankfurt plenty of times and I know where I am going. If you have to wait for luggage and don't know where you are going, that could be half an hour longer, or more.

But the real problem, as Frank says, is "if flight is on time". In a dozen flights to Europe, I've been an hour late three times. Once, the Lufthansa plane flew from Frankfurt to Denver to Frankfurt, with only a short turnaround in Denver. The plane was several hours late leaving Frankfurt. They made up an hour in the Denver turnaround, but we were still an hour late arriving at FRA. Once, on a flight from Atlanta to Stuttgart, we were an hour late because we were late taking off from Atlanta due to thunderstorms.

To where are you going by train right after arriving? Is it close enough to be using regional trains, with a regional "pass"? If you arrive on time and everything goes right, you could be at the closer Regional Bahnhof in time to catch a much earlier train.

And, as Chris-s points out, a Rail&Fly ticket allows you to leave on the next express train, whenever you arrive, at a price competitive with a advance purchase price.