Paul is right. You may not have to divide some of them for years, but your goal is to get as many blooms as possible over a long period of time, that means optimizing your plant growth to get the scapes you need and that means creating new fans. How large you want your clumps to get before you divide has to be taken into consideration also. You have been dividing some pretty large clumps, so you know how much work that is, if you let them get too large they are really heavy and awkward to handle.
A lot of people do not want large clumps, so they divide often. I like to have some large clumps just for show, but it is so much easier to work with smaller clumps when dividing. But that does normally mean not many or any blooms for a while. The way around that I have found is to have so many varieties you hardly notice when a few are not blooming.
