We have 3 window airconditioners in total - two RC, one cooling only, one of the RC ones is ancient and rattly and lost most of its plastic casing but still works perfectly, the others are 1 and 2 years old. Zero maintainence costs. You're *supposed* to change the filters every year but noone does and they just keep working. We took the filter in the one in our kitchen (one of those country dine-in kitchens) out to wash it though, before we had an exhaust fan over the stove the aircon was picking up the oil in the air and smelt like food every time we used it. All good now the exhaust fan is in, but this is one to watch - don't put ACs too close to a cooking area if your rangehood is a bit poor.
After quite a few years you might need a regas. Inlaws have a big split system that blows air around more than cooling it and it SO needs a regas but they just mutter about it ...