Most of the time, none.
I have two small (5500 BTU) Daewoo window units. I take them out in the fall. Last summer I didn't bother installing them back in the windows, and it wasn't too bad. I just vent the house at night, when it's cool, and then shut it up in the morning when I go to work. The extra insulation in the attic keeps the house fairly cool until I come home. I might put one back in this summer, depends on the weather I suppose. But it would probably be a better idea to finish insulating under the floor in the crawl space, instead.
PS-Double glazing is highly overrated, IMHO. The R factor of single pane window glass is about 1. The R factor of double glazed windows might be 2. But the area of the windows is relatively small, compared to that of the walls, ceiling, floor. You get the picture. Important to seal off any air leaks, which is probably the biggest benefit of getting new windows, but more economically done with weather stripping, caulk, and common sense repairs. From what I've read, the order in which to insulate is: ceiling (attic), under floor (if raised foundation), walls, and then finally, windows. In all cases sealing off air leaks should come first.