Must be quite a sight to behold! You can witness this at Körös River (Hungary), Central Europe, if you are lucky. Because of these kinds of occurrences being largely dependent on environmental factors such as temperature or moisture, it was nearly impossible to tell exactly when they would happen.
Life in the Undergrowth
Read more about the amazing documentary here.
Since mayflies have no mouths nor stomaches, they rely solely on fat reserve for energy to fly. That means a flight time of roughly 30 minutes.