A grease trap does nothing once it is full. Cleaning frequency is set by how fast the particular kitchen fills it, not by a rule, which is why a schedule that worked in January can be wrong by summer. The service itself is a licensed waste collection, and the paperwork it produces is as much a part of what you are buying as the empty trap.
Setting the frequency
Contractors normally start on an estimate from the kitchen's covers and cooking type, then adjust after the first visits show the real fill rate. A trap that is more than a quarter full of grease and solids at the visit is being emptied too rarely.
What a proper service includes
Removing the accumulated grease and solids, cleaning the baffles and the sides rather than skimming the top, checking the unit for damage and correct operation, refilling where required, and leaving a written record of the visit and the volume removed.
Cleaning one yourself
Small under sink units can be cleaned by kitchen staff, and many are. The grease still has to leave the premises lawfully rather than going into a bin or a drain, and the record still has to exist. Larger and external units are a tanker job.
The documentation
Each collection produces a waste transfer note naming the carrier, the waste and the destination, and it has to be kept. Environmental health, the water company and a landlord's agent will all ask to see the last year of them.
Questions people ask about grease trap cleaning
How often should a grease trap be cleaned?
As often as the kitchen fills it, which for a busy site can be monthly or more. Start on the contractor's estimate and adjust it against what the first visits find.
What happens if it is not cleaned?
Grease passes through, the drain blocks, and the water company can recover the cost of clearing the sewer. It is also a finding at a food hygiene inspection when it affects the premises' cleanliness.
Can I use a chemical instead?
Products that emulsify grease move the problem downstream rather than solving it, and water companies discourage them. Physical removal by a licensed carrier is what evidences the duty.