Bodark,
It is not possible to give a good answer to that question.
In addition, to the particular conditions at your pond, the daily wind activity is another important variable at Theo stated above.
Finally, the time of the year is probably the most important factor. Due to the annual temperature cycle, your pond should be relatively well-mixed in the spring and fall, and peak stratification should occur in the summer and the winter.
It is probably peak stratification during late July and early August that is your biggest risk since the warm water holds less dissolved oxygen.
In your situation, I believe experimental data would be the best solution. Shut off your aeration system and sample your pond over the following few hours or days. Unfortunately, professional level oxygen sensors are quite expensive. There are cheap ones at Amazon ($90) but they do not have a long probe cord, so you would have to buy or create a deep water sampler.
I believe several of the fish suppliers that post on Pond Boss have their own probes and would probably test your water during a delivery, but you would never be taking fish during the hottest time of the year.
Sorry to provide such little useful information!
You might be able to run your own "poor boy" experiment. Shut down your aeration on purpose on the hot days and observe your fish. They will let you know if the pond oxygen levels are getting dangerously low. Make sure you have some charged batteries to immediately re-start your aeration since this test would be more accurate during a wind calmed period.
Hopefully you can install a reasonably efficient system within your budget. Good luck on your project!
P.S. I suspect you are a long way from AC electricity. However, airline costs are much cheaper than running electrical wiring. Consider that trade-off as you start pricing your windmill aeration system.
