Ideally you drain window units every couple of weeks, when you clean the filters. Once a year a deeper cleaning is a good idea. One where you take the cover off, clean all the built up gunk out, and then let it dry. You can use a cleaning spray specific for air conditioners, or even just water for a significant improvement.

I typically use the cheapest “no rinse” cleaning spray for air conditioners that Home Depot sells, a toothbrush, and lots of water before/after. It really extends the life of the unit, preventing the build up of mold and improving the units efficiency.