Sounds like a lot of passes, I don't count (just go off feel) but sounds like a lot. (usually you can feel and hear when it's smooth to move on)
I'm guessing it has more to do with moving to the next stone before ready? E.g. if you move to the 400 before the 200 has done it's work you'll wear out the 400 more quickly.
Similar, if you are filing your edges and don't use a smooth (2nd cut file) after the course cut....your 100/200 grit stones are going to take a beating.
The 100 (I don't use as I find it too aggressive) or 200 grit is going to get worn out the quickest by far. This is normal considering how much you'll use it and what you ask them to do. Finer grits are really just polishing an already smooth surface so won't see much wear there.
Maybe you are pressing too hard? There should be very little pressure on the stone during passes, let the tool do the work.
Another factor is the quality of stone, the "economy" stones don't last nearly as long as the "pro or world cup" stones....cheaper sure but I find they last half as long.