I recall (unless I'm confused) that p2pool uses a PPLNS window of N=3 blocks, so if blocks are found frequently, shares will last 3 blocks of time. However when blocks are taking longer to be found like at the moment, is it 3 days before shares will expire off the end of the max sharechain size? I couldn't quite remember if it was 3 days or some other period of time.

Each share contains a generation transaction that pays to the previous n shares, where n is the number of shares whose total work is equal to 3 times the average work required to solve a block, or 8640 (= 24 hours of shares), whichever is smaller.