Under normal circumstances, the impellar should last for years. It's very easy to access, making it a good idea to clean it whenever you open up the canister filter to change or rinse media. I think the shaft is ceramic, so it's important to be somewhat cautious in order to not snap the shaft by torquing on it too much. Similarly, if the shaft's metal, it's important to not bend the shaft, as that would create problems as well.
Good luck with your 2217. I've really like the 2215 we have... It's been a nice little canister filter that's done what I've needed it to do.