Well, it‘s really difficult to tell how much time you‘ll actually need, as that will depend on several factors that cannot be predicted too well. Of course, you should always factor in possible delays (particularly after long-haul flights), how much time it will take to go through immigration and how long the baggage reclaim will take. The entire process might be as short as 30 minutes, or as long as one and a half hour (though unlikely).
Once you are landside at Terminal 2, you will have to walk towards the people mover (or, alternatively, the bus, which stops nearer to the railway station but doesn‘t run as frequently as the APM), take the short (<5min) ride, walk across check-in hall B, up an escalator and then the approx. 300-400m walk across the walkway above the highway to the station. If you‘re really quick, you can do all the walking (including the APM ride) in about 15 minutes, but be warned that there‘s really a lot of walking necessary in order to get to the station.
Depending on where you need to go by train, my advice would be to book a flex fare ticket (as opposed to „Sparpreis“) which is significantly more expensive, but will allow you to take any train that goes directly to your final destination. For example, trains to the direction of Cologne will depart roughly every 20 minutes, trains in direction Mannheim-Stuttgart/Karlsruhe will depart, depending on the hour, once or twice hourly. I would say that this is probably your safest choice.
However, if your flight will be on time and everything else works out well, I would say thar 2 hours is enough to make the connection.