The micro mesh pads lasted one year for me. I can't tell you how many pens. Right now I am using the other pads. They are not lating as long as MM. I have done in the last three weeks nearly 100 pens and 56 wine stoppers. I be going out to woodcraft to pick some more mm.
This past May I bought the MM set that Woodcraft sells. The first thing I did with them was to cut them and the pad in half. For penmaking, I think, this is a good size to work with. I'm currently using only one set. They get a bath weekly and they look/work just fine doing 20-25 pens a week.
I'm still on my first set of 2" square foam padded ones I got at a WW show ... either this past april or october '05. They've got probably about 15-20 total hours of sanding time on them ... they melt real easy if you use them at too high of a speed. You need to slow that lathe WAY down when you sand 'em. WAY down. You can't let 'em get too slow.
I don't know how many but it is hundreds! I am lucky enough to have a local store that sells all the individual MM sheets so I can occasionally replace the finer grit ones.
As Chris mentioned...a set (which is only a small part/slice 1 x 3 in my case) will alst atleast a hundred pen IF treated properly. A very cheap way to attain superb finish.