The timing of septic tank emptying has no fixed rule of thumb. The speed at which solid waste at the bottom of the tank builds and the scum floating at the top of the tank take up all available space will determine when the tank needs emptying. Other factors used to determine when to empty the tank include the number of people living in the house and the size of the septic tank.
If you empty your septic tank once a year you may never have a problem with waste materials leaking out. If only one or two people live in the house, emptying the tank every other year may be just as effective.
You can buy enzymes to put into your septic tank that will break down scum and solid matter. If you use enzymes monthly, the septic tank may not need emptying as frequently.
If you use a professional septic tank service, you may arrange for an on-demand emptying or a routine emptying.
Don't wait until your septic tank is blocked before you emty it. You can prolong the life of a septic tank with routine cleaning and careful use.