How I clean my drop pans: Clean drop pans with "the works" toilet bowl cleaner, water, and sponge. The works can damage your skin so take care. Rinse thoroughly. Coat entire drop pan with a sprinkling of baby powder(the corn starch based one). Throw in each back corner a handful of equine pine(pelletized pine bedding). Top with a layer of pine shavings.
I normally clean drop pans at least once a week.
I usually do a deep clean once a month. This include things like scrubbing down the whole wire part of the cage, soaking water bottles, soaking toys, cleaning the area the cages are set down on,etc. Any thing that the rabbits can come in contact with needs to be thoroughly rinsed as to not harm the rabbits of any cleaning chemicals.
Not all rabbitries have drop pans in a stacker system set up. Some people have hanging cages that allow the dropping to fall, then simply rake them up . This method takes up more space but makes it much easier clean up.