Navigating Frankfurt Airport

I always arrive 3 hours prior to my international flight. That said, we traveled by direct train (50 minutes) from Cologne for a 2pm flight. Arrived around 10:30. It’s a bit of a hike from the train station to the check in area (10-15 minute walk). There was no one in line when we checked our bags. Then we went through immigration. I don’t recall that wait being very long, maybe 10 minutes. Then security took about 20-30 minutes. Then it was a ways to our gate. We had plenty of time to get lunch before boarding.

The two hour window would make me very nervous. Trains can be delayed, and if you miss your connection in Mannheim, that would be very stressful. Give yourself plenty of time to make that connection.

You will need to take the free shuttle bus to Terminal 2. This is not a walkable distance, as there are no sidewalks. You will need to walk from the long-distance station to where the buses stop at Terminal 1, which is at B.
When you get off of your train, go up the escalators to the next level, follow the signs to terminal 1. This is a straight walk down a passage. When you get to the dead end, turn left and go down one set of escalators. Here, there are doors on your right. Go out of these and you will basically be where the bus stops. There will be signs.
There is no customs when you depart a country, only security check where they run you and your bags through an x-ray and look at your passport quickly. They won't be opening your bags or anything. If there is no line, this is quick.

For Frankfurt, I would always plan on getting there 3 hours before an international flight. Always. They have staff shortages here just like everywhere in Germany.

Frankfurt airport website provides answers for terminal change.
Terminal map: https://www.frankfurt-airport.com/en/airport-guide/orientation/airport-map.html
How to find way from Fernbahnhof: https://www.frankfurt-airport.com/en/transport-and-parking/by-train/long-distance-trains-of-german-rail.html
Terminal change by bus or train: https://www.frankfurt-airport.com/en/flights-and-transfer/transferring-at-fra.html

Walking is no option.

Your airline defines the window how long in advance of departure you shall be in their terminal.
Normally 150 mins shall be enough.

When you depart a country there is check-in with luggage drop and then only security and passport check - no customs.

I would plan to arrive by train no less than two and one half hours beforehand. Depending on passenger traffic the lines for coach can be very long or they may move swiftly. I’ve seen both scenarios. I’ve arrived and departed the airport by train and bus and easily navigated to terminal 2 for Delta.

The trains arrive near terminal 1. A shuttle bus is readily available. I took the Skyline (monorail) which takes just a few minutes because I was conducting business in both terminals. Directional signage was very clear. Below is a link describing the options. There are other YouTube videos that provide detail walkthroughs.

Unless something has changed since my 2022 travel there you deposit your checkin bag upon arrival before security and passport control. You may encounter two different security checkpoints. There is/was a very active secondary screening station at the boarding gates for U.S. international departures for individuals with SSSS stamped on their boarding passes. I along with quite a number of individuals were selected for this process. Staff were very professional and efficient.

https://youtu.be/pzDsELhuGpo?si=aFVROqRsZFIgwZQF

Thanks everyone for this helpful information. We'll plan on taking the earlier train from Heidelberg to give us plenty of time to reach our departure gate and hopefully have a little time for lunch.

The trouble with the bus from Terminal 1 to 2, is that, at least according to the Bahn website, it's not a dedicated shuttle on a regular schedule. It's regional buses that go through the airport on the way to/from other towns, so the schedule at the airport is not regular. Sometimes there is only 2 minutes between buses, but at other times it can be as much as 26 minutes, or even 45 minutes. I think I would take the SkyLine between T1 and T2. It takes a little more time to get to it, but is runs regularly, every few minutes. You are less likely to have a long wait for the next bus.

My first trip to German in this century, was with NW Airline, which merged with Delta. Like Delta today, they used T2. I came into the airport on the S-Bahn from Mainz, went through T1 to the SkyLine station, then took the SkyLine to T2. NW had said we should arrive at the airport 3 hours before flight time. I arrived 2¾ hours before flight time. The line was enormous. Because it was the only NW flight to the US (DTW) for the day, they only had two people checking in an entire 747. After I arrived, I don't think more than a couple of people came in behind me. I finally got checked in an hour before flight time. Had I arrived an hour before flight time, I still would have been checked in at about the same time.

Lee, there have been a lot of changes at the Frankfurt airport and one of them is that there are dedicated shuttle buses that run every 10 min. between the terminals, on a set schedule. They have been running these for quite a long time.
Relating experiences from a decade ago are not very helpful.