When you stop mining, the amount of time you spent on getting nothing (wasted) will on average be half the time for you to find a share unless you plan waiting and stopping WHEN you find a share.
For P2Pool the amount of wasted time will of course average out to be the P2Pool difficulty times more than a 1 difficulty pool for your hash rate.
That would be true if you worked TOWARDS a share. However you don't each hash is independent. Randomly turning your computer on and off wouldn't affect your payout on 1 diff shares vs 1000 diff shares as long as the "on time" is the same. If you were on 50% you would in the long run earn 50% regardless of the share size.
Nope, that was the point of my example.
It's to do with when you stop mining and the average expected amount of time you will mine and gain nothing for it.
(which will be - on average half the average expected time it takes to find a share)
Edit: yes of course if you resume mining the total expected shares is still based on the time you mine - I'm just referring to if you stop mining.