I was replacing mine every 1 to 2 years, so probably around 3000 miles or so. My problem was that I would start to slide forward on the saddle and it would bother my arms and hands more than anything. Then once I realized what was happening, I would notice it sooner and eventually I just got frustrated and changed saddles. I had less problems when I rode in the flatter lands. I think that rocking back and forth a bit more with climbing accelerated the breakdown.
Living life like there's no tomorrow.
http://gorgebikefitter.com/
2007 Look Dura Ace
2010 Custom Tonic cross with discs, SRAM
2012 Moots YBB 2 x 10 Shimano XTR
2014 Soma B-Side SS