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.