I have a window unit like you describe as well and it leaks light like crazy. Not much of an issue in the winter when it gets dark around 6 o'clock, but in the summer when it's still light outside after 8 P.M. it's more of an issue. It's surprising how much light leaks in my darkroom, it's an old motorcycle shop I converted about four years ago. I've done a paper fog test during the winter months when the AC unit is covered, and although there are lots of leaks other places, I've experienced no fog. During the summer, I hang a sheet of dense black cloth from the rafters to separate where my printing station is and this helps. I also use a Thomas safelight and think this helps, although I have no scientific basis to back my claim up. I used to worry a lot more about light leaks than I do now.