In reply to Pay Attention:
I've got a written list of everything I've bought and when from my early days - more recently, I have a box of the tags with the receipts stapled to them.
It's mainly for insurance - if I have to claim for lost/stolen gear - but also handy when I used to do a lot with Scouts and was using my own gear for setting up anchors. We certainly had a log of use for every bit of gear, but you can only really log number of days they were taken out climbing, rather than actually used (I might not use all of the slings on a given day, for example), unless someone takes a big fall, in which case the bits of gear involved would be removed from use.
I could cross-reference my gear against my logbook for an "estimate" of use - not every bit gets used on every route.
Mostly, it'll get replaced on visual inspection, but I might check the age on software and go from there.