The Environmental Protection Agency recommends inspecting and pumping your septic system every 3 to 5 years. Septic pumping frequency depends on the number of residents, the amount of wastewater generated, the volume of solids, and septic tank size.
Professionals should also inspect the system every 3 years to look for leaks and layers of scum and sludge. The tank needs pumping when the bottom of the scum layer is within 6" of the bottom outlet tee, or the top sludge layer is within 12" of the outlet tee.
However, if you suspect your tank is full, don't wait until then to call a professional.