going through customs in Frankfurt. how much time to get a train?

You can't be sure your plane won't be delayed, your luggage not mislaid and there not being a long queue at the Immigration. Play it safe.

Customs is always a walk through the Green door with nothing to declare. Immigration can be the hang up. Depends on number of officers on duty, number of planes arriving at the same time, etc. The Germans are pretty efficient but an hour is pretty tight. I would allow for more time because you cannot control several variables. Even the time getting off the plane depending on your seat location could be 15 to 20 minutes after the door opens.

Your major bottleneck is not customs, but immigration (these are separate stations in Europe). I have never found immigration in Frankfurt particularly back-logged, but if several flights arrive within a short time frame, you may face delays.

You will almost certainly have "nothing to declare", so customs will be simply walking through a gate.

Non-stop express trains to Nürnberg depart all day, sometimes more than one per hour. If you're thinking about advanced purchase discount tickets, I would pick a later train, probably no earlier than 10:00. If you don't mind paying the full price of €58, just buy the ticket at the station and hop on the next train.

Express trains depart from a different terminal and you need to be bused from the international one; You need to plan on at least a 2 hour difference between the flight and the train, maybe a little longer if you are checking baggage.

stephen,

IMHO, that's not enough time. As the others have said, there are a number of factors that can cause delays so you may not be able to connect with a train that you've purchased a ticket for. In addition to your flight being delayed, if you're seated in Economy and are one of the last to de-plane, you may find a huge queue at Passport control (depending on how many officers they have working at that time). In addition to that, FRA is a large airport and in a jet lagged state, it may take you some some to get to the rail station.

Note that FRA has two rail stations, a Regional Bahnhof for local trains and a Fernbahnhof for long distance trains. Did you note which station you'd be departing from?

Buying tickets to connect with an inbound flight is not generally recommended by most here for the reasons noted above. I agree with Tom, the easiest solution is just to wait and buy a ticket for the next train when you arrive at the airport rail station.

We just were at Frankfurt in September. Our flight arrived about 8:15 in Terminal 2. We caught the bus to Terminal 1 which drops you close to the train station. We did not have tickets when we arrived around 9:30., so we bought tickets and caught the 10:00 train. I think the hour and 15 minutes might be a little close. The price may be a little higher buying the ticket that day but you will have less stress trying to get to the train station to catch an early train.

If you tell us which airline you are using, we can tell you which terminal you will arrive in. This helps in planning your logistics. Usually in Frankfurt you will be emerging into the main part of the airport about 45 min. after you land. It is pretty efficient. Show your passport and walk out.
You can check the punctuality of your flight by Googling your flight and #. It will come up on Flight Aware. Some flights, depending on where they depart from, are often late.

The train tickets bought on the day of arrival aren't that expensive, so if it was me, I wouldn't purchase them ahead of time. Save yourself some stress.

I don't understand exactly why you are terminating at FRA when there are connecting flights on to Nürnberg.

But if that is what you have already ticketed, you have several options. I would not purchase a non-refundable ticket to Nürnberg for so close to my scheduled arrival time. I've usually been at the Fernbahnhof within an hour of landing, but I've been an hour late landing on at least one of five arrivals.

1. Just buy a full fare ticket by ICE to Nürnberg. It will cost somewhat more, 58€/person, vs whatever Saving Fare ticket price you can get, but you can just get on the next ICE to Nürnberg.

2. Purchase a Savings Fare ticket for a couple of hours later as a buffer to allow for a late flight

3. Take regional trains. Regional trains to Nürnberg take less than 4 hours vs a little more than 2½ hours for the ICE. You take the next S-Bahn to Frankfurt Hbf, then one of the hourly REs to Würzburg and on to Nürnberg. You'll need to buy an 8,25€/P local RMV ticket to Kahl am Main, the first stop in Baveria and a two person Bayern-Ticket (28€) for the rest of the trip, 44.50€ total. The extra time for the regional train is probably offset by the time buffer for using an express train.

Express trains depart from a different terminal and you need to be
bused from the international one

This statement makes no sense. What do you mean by the "international one". There are buses that go from the airport's Terminal 2, which is not exclusively" an international terminal". Terminal 2 services international flights from airlines other than Star Alliance airlines plus non-Lufthansa flights within Europe, The buses go to Terminal 1, which handle Star Alliance international flights and Lufthansa flights within Europe, but you can also go via the SkyLine shuttle between the terminals. From Terminal 1 you walk out to the Fernbahnhof, which handles express trains, past the Regionalbahnhof, with handle regional trains and the S-Bahn to Frankfurt.