Like they've mentioned every county, every state, every department, is different. Count your blessings.
We have an above ground one, probably 36x36x36 or so--I'm not sure we'll have to drain it but for every few years. It does have some rancor associated with it after a day off. Rancid butters and creams I'm sure. Nothing else I know that could create real stink. It's not like we're frying in here.
We use a bio-solution that helps digest fats and keep everything running smoothly. It is recommended for those with drains that are needing to be kept clean.
http://www.newtechbio.com/greasetraptreatmentproductinfo.htm
It makes the place smell rather nice after cleaning but since we didn't have a problem I don't know of its efficiency I'm just taking steps before I might have a problem. The longer I don't have to open the hatch of doom the better. I can only imagine having to clean this unit out and the thought is rather unpleasant.