They reckon a good bat will last 1000 runs, its not a firm set rule as the SCAT has done 2500 but has now been retired due to war wounds that mean its served its duty.
With wood being a natural material you cant say how long it will perform for or last, with the bats the Pros have they want maximum performance and arent worried about longevity due to the fact they arent paying for them, they are therefore pressed softly or to gain peak performance. Where as we as club cricketers have to spend our hard earned coin on them so would like them to see us through a few seasons at least ideally.
If the bat is well looked after (cracks repaired, well oiled and stored correctly) then there is no reason its performance shouldnt continue over time. We have to accept the inevitable fact that when you buy a bat at some point its going to break....the question is when!