The longer you wait before rank increases the more attached someone becomes to their items.. resulting in it being harder to raise the caps...

6 months 5 ranks is a good amount.. 5 ranks doesn't kill old content, but keeps things growing. if you wait 1-2 years then raise it 10-20 ranks, you get the reaction of you just killed off X,Y,Z... because they were too relevant for the longest time.

If you are constantly on a fixed time increasing the ranks, people know, dont get attached and will be ready for the next content when it comes.

6 months, 5 ranks is what I feel would be best... gives even the casual player enough time to rank up 5 ranks in the 6 months time on a few classes.