http://www.montana.edu/wwwpb/pubs/mt9403.html

P.S. On that page:

Table 1 lists how often you need to pump out your tank on average,
given the size of the tank and the number of persons living in the
household. These figures were calculated assuming there was no garbage
disposal unit hooked up to the system. The use of a kitchen garbage
disposal will increase the amount of solids in the holding tank by as
much as 50 percent.