I have a 12000 BTU window unit at home. I have to clean it twice a year or it gets all gummed up from cigarette smoke and then dust sticks to it. If I don't clean it, it will run the compressor more and more to maintain temperature until finally the coils ice up. Not good for the electric bill.
I have to take it outside, take the cover off and spray it down with degreaser then get a toothbrush or similar and scrub the coils then hose it all off.
They also make a product called coil cleaner that serves the same purpose
Edit:
I forgot to mention that you don't have to be a smoker for the unit to get clogged up. Oils from cooking (and probably other sources I can't think of right now) will eventually do the same thing