I've had my P3P for a few weeks and done around just over two hours flying - apart from chips, cracks, half dog eaten props, and reduced life if you have to take props off for transport and storage - is there a maximum flight time that props should be used before one should use a new set?

When you start to feel the threads failing.. I remove my props after each use, I still use the original props on both of my p3's with hundred of flights. The key is to not over tighten them..
Please replace ANY prop that has made contact with anything, even a small twig. Even if the prop looks fine after the impact it might have weaken them.

I have 120 flights, 18 hours, and over 60 miles of flight since May, and still fly on my first set.
If they are subject only to aerodynamic load, i.e. you have not crashed, or bumped into anything, then I don't think that they are subject to a fatigue life.  
Just inspect after every flight for nicks in the leading edges, scratches on any surface, or damage of any kind in the threads.  Also by flexing and twisting the props you should reveal any latent cracks, but be gentle.

I did start a thread for any fliers with more than 100 flights and no crashes, to report any defects or failures in any part of their P3's but got very little response from the community, so these views are my own.  If you want to see the results from the 10 replies that I did recieve, then search the forum for "100 flights".
I hope this link works to save your search:  http://forum.dji.com/thread-30038-1-1.html  it may reassure you.