What newcomers to p2pool usually don't immediately grasp is there are two types of shares in p2pool.  The type that you see your miner submitting all the time, and the type that really count. It's the second type that you aren't getting many of.

To elaborate, p2pool uses an "alt chain" of shares to keep track of who's submitted shares.  You can consider that "alt chain" to be like another coin, with a much much lower difficulty.  p2pool adjusts the difficulty required to get on that chain so that miners are getting shares on the chain roughly 30 seconds.  (I think that's the number, the wiki might be outdated).  That means the more people there are mining, the faster shares are found, so difficulty goes up.

Right now that share difficulty is at 1,809,778.  Your 171gh/s needs to find a share of that value or greater (at the time of this post), to get on that chain.  That's why it's estimating it'll take you 12.6 hours to find a share.

The good news is once you get a share, it's good for 3 days.  So, at current numbers, making it 12 hours per share to make it easy, you should have 6 shares on the chain at any given point, assuming average luck.  That's where variance comes in ... sometimes that number will be higher, sometimes that number will be lower.

M