The answer is "it depends." A tank should be inspected every 2-4 years (more often if you have a garbage disposal), but frequency of cleaning depends on the size of the tank, the condition of your system and the use it is given. We generally recommend a cleaning every 2-4 years.
If the solids, which settle to the bottom of the tank, are not broken down by bacterial action and build up to a high level, they can be carried from the tank into the leaching system, clogging the pipes and the field. Eventually the remaining undigested material accumulates and must be cleaned out. This material is called "sludge" or "septage."
Connecticut State Law requires a minimum capacity septic tank of 1,000 gallons and 1,250 to 1,500 gallon tanks for a four or five bedroom house.
At a use rate of 60 gallons per person/per day - considered average by most plumbers - a family of five uses 109,500 gallons of water a year! If the number of people in the house exceeds the average for the size dwelling (five people for a three bedroom house, for example) - or if the septic tank is smaller than the state regulations now required - the system should be cleaned every two years until experience indicates a longer period can be allowed.
Sludge accumulation is approximately 75 gallons per capita per year or 300 gallons per year, per family of four. Check with your Town Sanitarian or Director of Health for information concerning a house's septic system.