Each municipality requires the restaurants on its sewer collections system to follow different maintenance schedules. Durham and Orange County require a grease trap to be pumped out every 30 days. Carry requires every 60 days. Wake County requires grease traps to be pumped before the grease and solids build up in a grease trap acount for 25% or less of the total volume of liquid. Here is a good source of pre-treatment ordinances for various municipalities in North Carolina.
A good basis would be to have a pumping frequency of every 60 days to start. If the indicated solids and grease amounts on the manifest left by your pumper after the service are lower than 25% grease plus solids, then you could move your frequency out to pumping every 90 days. If the number is greater than 25%, you should increase your frequency to every 30 days.
