I know of people who have churned through full size type 30 stones like bench nagura and suita in about a year - so your datapoint seems quite accurate there. The carpentry folks not only buy most of the really high end suita but are more than capable of using them up in relatively short order as well.
For a hobbyist, a harder stone at 40mm is almost certainly a lifetime investment, even doing many projects a year. On my harder stuff I haven't detected any noticeable change in weight or height and I do a fair amount of polishing. The soft stuff is a different story. In aggregate, I've shaved through at least 30mm of soft stuff like Akapin, soft uchi, tenjou suita, maruo shiro, etc in the last 2-3 years. Besides coarse synths these are the place I'm spending the most time working though. I also do a decent amount of polishing, at least an hour a day on average, probably a good bit more. Add in loss for removing layers of su, etc.
All that said, if you're using your stones enough to wear through them you're almost certainly not going to be bothered buying another at the end. Stones / polishing on an $ / hour basis is certainly cheaper than a lot of hobbies, just with a high initial cost.