Fortunately, Frankfurt Airport has made a "Transfer Guide".
It describes how to get from each terminal to the others, using their Skyline train, Eshuttle and walking.
In addition, it points out that if you have less than 30 minutes for your connecting flight, you may use the fast lane at the security checkpoint, giving you a bonus speed option to get through if you're worried about passing those points.
It indicates maps further on in the guide, so if you can work out which terminal you arrive at, you can probably work out roughly how long it will take you.
Then in terms of your flight, we can look at the rating for Frankfurt airport on Flightstats.com: 3.6/5.
Recent comments also indicate people get very frustrated with transfers here, however it's prudent to remember that happy people are less likely to comment than complainers.
They also have a scorecard for different times of the day, airlines and so on, so you can get a rough idea of how likely your flight is to be delayed.