I've had to replace a few of mine. Never kept count, but if I had to guess probably in the 50+ range. Could be more. But I'm also not someone making 200+ pens for large craft shows frequently.
For me, it also depends on the sanding. For acrylic pens, off the tool I can start wet sanding at 500. For wood I dry sand through the grits usually starting at 120. That sanding will take it's toll on the bushings too. Since most of my requests are acrylic, the reduced sanding extends the life.
Bunch of people on the FB pen turning group I'm in swear by getting rid of bushings and just using calipers while turning between centers. I could do it if I had to, but using the bushings makes it faster & easier. I did go halfway as I got some TBC adapters for the bushings and ditched my mandrels.