These plastic parts have only so many "cycles" in them before they fail. The trick, or so it seems to me, is to replace all the plastic parts once one of them fails. I may be wrong but I think the correct measure would be to say how many "days" or nights in the rig as to say when to do a total refurb ($70 or so dollars. Amazon) as opposed to measure the failure in time. Obviously, full timers would put those plastic parts through more "cycles" than a couple that use the rig 20 nights a year. Thus requiring service more often. The long-short is that stuff just wears out - some things faster than others. That Thetford Style II was easy to remove, replace parts and reinstall as part of a repair/maintenance program.
