With few tests it measured that wind mill at maximum height generation is about 3-6 (without rain) or 8-10 (with rain/thunder)They WILL break much more frequently, but iron loss can be minimized by placing some water and obsidian pipes (or RP transposer) below towers.
Whenever a wind mill breaks it drops iron and it will fall into water (some into cables unless if you make the collection system above them) then collected by BC/RP system and sent to a chest ready to be used to repair them.
OK, look at the generator and at the wind mill, they are almost the same thing. They have the same metal looking color.
Since its turbine face random directions, it is annoying to detect broken wind mills.