Fly in the House

Hello, We seem to have a resident fly. How long can a fly live? This fly is larger than a fruit fly, but smaller than a house fly. We do not have a species name for it. We have seen it for several days. Flitting around nervously. One day it was sitting on a window, but behind the blind. Raising the blind caused it to fly away. Another day it was on the table, however the fly swatter was on the side away. Walking around disturbed that flighty fly. Today it was on the counter. I managed to get the swatter, but missed on the swat. Does this fly lead a charmed life or is it reproducing, but only one at a time? We have never seen two flies at the same time or in the same room. This fly hangs out in the kitchen, but once in a while it ventures into the bathroom. How did it get here anyway? It is still winter and not the season for flies. We do not have screens on the windows so have not opened them even on the warmer days. I have been thinking that I should get a few screens from the attic, clean them, clean the windows and enjoy a nice warm breeze when it comes. I did open the kitchen window when I saw that fly close, hoping it would just fly out. To encourage that motion I flapped a towel at it. It has not been seen for some time now, perhaps it did fly away. Paula