I think I'm shifting towards replacement as an excuse for an upgrade! I always used to hang on to the existing stylus until I thought it had had enough. This was usually about 18 months - 2 years for the Goldring 1042 with its line contact stylus. I stuck with that for quite a few years.
When I changed to MC with an Ortofon MC15, I was tempted to exchange it for an MC25FL after only a year or so. However, replacing a stylus at that level costs more than I'm used to, so the MC25 had probably had something like 2 years' use when I exchanged it for a Rondo Red, at which point I had the impression it was due for a change. Plenty of life in the Red still, but I am already feeling tempted to go up the range.
OTOH, the Red sounds really good!:D