I would expect to get at least 5 years wear from a pair of good quality bibs which is why I'm always happy to pay £100+ for bibshorts which suit me. That's around 15000 miles - I usually have two pairs on the go and use them on alternate rides. I ride about 6000 miles per annum.
The area which tends to show wear is at the base of the spine where the lycra becomes transparent. When your mates complain they can see your backside it's time for a new pair. I check this from time to time by stretching the fabric over my hand.
I've been wearing Assos for the last 20 years, probably longer. I've a couple of pairs which are 10 years old which I still wear underneath biblongs in winter.