A septic tank should be desludged whenever sludge builds up enough to reduce the tank's settlement performance, which for a typical household is generally at least once a year unless a competent inspection shows otherwise. Desludging must be carried out by an authorised waste collector and records should be kept.
The correct interval depends on tank size and the number of occupants, but the principle is that sludge and scum must never build up to the point where solids can carry over into the percolation area. Once solids reach the percolation area they clog the soil, and the resulting failure is expensive to repair.
Sludge is a waste material and must be removed by a collector authorised for that purpose, then taken to an authorised facility. Keep the receipt: inspectors under the National Inspection Plan and purchasers' solicitors commonly ask for evidence of maintenance.