Always more than one would think. Nothing sweats more than a beer or soda can. When I plug the drain in mine, I can get an inch of water in no time. Add any organic substance and it does stink. Even if you don't spill, things may have spilled on your containers in the factory or store. The condensation washes it off and it seems to like to grow on boats..........
I have my drain hooked to a bilge pump with a bypass valve. I leave it on the "freezer" side most of the time. Just manually turn the pump on once and a while. I trickle a little bleach in once in a while to keep any smells at bay. Any serious cruising i switch the valve over in case i need the additional pump. Factory installed, kinda nice. Keeps everything nice and dry below.