I have a wintec that is flocking that I have had for 10 years and never had re-flocked. It has changed shape over the years and is now very flat through the panels, but it suits my flat backed horse so not an issue!
Really, you will only need to adjust the flocking if it becomes a fit issue. I just bought a custom made stock saddle with real wool flocking under a felt lining. I ahve had it maybe six months now and soon i'll take it back to my saddler to re-evaluate. He has a contraption that looks like a spike that he uses to fluff up/move the flocking around through the felt - He just gets to work stabbing my saddle, lol! Because it is now worn in, once he checks it over I probably won't need to do anything for quite a few years unless my horse changes dramatically.