I will tell you what prompted me to do mine just recently. As usual while underway, I went down below while motoring to make my rounds. Bildge was dry, no unusual noises, belts all tight, no oil or diesel leaking. Hmmmm, the stuffing box sure does feel warm. Not hot but warm on the warm side. The seawater cools the shaft and the flax prevents leakage. I didnt like the warm feel. So I changed it out. The flax was shot and I believe as it breaks down it prevents dripping because it breaks up and "clogs" the front of the shaft nut. I still had some adjustment left but it was more like really dripping or a half turn or so to dry. Not normal. Mine just like Rich's was in the four plus year range. I dont know, but I think the heat test may have some merit.
Tip: A small slotted screwdriver, I think, works better than a flax pigtail because it digs and gets that last ring out. I believe typically there are three rings of flax to dig out.
Cheers