I think it may depend on the size of your fans motor. You might want to read your manuel, call the manufacturer, or the store where you bought it and ask them!!
I don't know!
I agree with midlifemadness.
And you should also look closer to what your fan is doing. Ask yourself questions like: is there wear, cracks, is something heating up, is some part getting dirt in it and look for all deviations from new. That would be about +20 deviations if you've had the thing for more then a year.
Yeah, I think it can burn out faster if you keep it on high speed all the time. You get more out of it at low speed, except for the crisp breeze, and are less likely to wear it out.