Yeah it's guranteed to be two hours long. I think Tf3 will be the longest becuase it's going to have to wrap-up everything.
How on Earth does that translate to 2 hours and it being shorter? If anything, the final movie in a series should be longer as it will have to wrap up everything. Hell, that's why the last HP movie is being split in two! We don't know anything about the plot. We don't know anything much about the set peices or action scenes. How can we take a guess at the film time?
If ROTF was 2 hours and 50 minutes long, then TF3 will probably be a little over 3 hours (if the TF movies keep getting longer).