Every household is different. The easiest way to figure it out is to use the following formula: an average family of 4 with an average 1000 gallon tank should pump it every 4 years. Of course this number varies depending on the number of people in the house and the size of the septic tank.  For example, If there are more people in the house, it needs to be pumped more often.  If the septic tank is bigger, then you can go longer between pumping.