About 10-15 years? Location has everything to do with it. Its all about the mindset and long term investment, do you have $25K sitting in the bank doing nothing which you can use to shave $120+ per month off your utility bill? That $120/mo. could be applied somewhere else over the life of the windmill incerasing your rate of return.
I do not think I will live another 10 or 15 years since I am already a old goat.