Heuristics for train delays: Does the train pass Frankfurt? Add 15-30 minutes. Otherwise expect your train within 5 minutes of its scheduled arrival/departure.
I lived in Frankfurt for a year and now I live in England. I was so frustrated at DB and RMV when my trains were a bit late (3-5 mins for RMV, 15-20 for long DB) once in a while. Now I take a regular train from my town to a city and I've literally only caught it on time TWICE in two years. Usually 20-30 minutes late or suddenly cancelled. Trains here are a whole different level of bad.
- Get as few train changes as possible. (Being a few minutes late is no issue if there are no connections to miss.)
- Any changes should be in big hubs. (You might be offered a route with a change in a smaller station, but if you miss the connection the next train might be hours off. If you change in a hub there will be many others.)
- Don't take the last connection of the day for the last leg of the journey, you might miss the connection and be stranded. Even worse if it's a small station (offices will be closed if there are even any there, there will be services like hotel and taxi somewhere but have fun finding them).