If you have not pumped your septic tank in several years, but do not seem to be having any problems, it may mean one of several things:
There is minimal water use in the home, and/or the size of the septic tank and the biological activity maintains the solids at sustainable levels. This is rare but may occur when there are only one or two people in the home.
The tank has a leak and is discharging wastewater into the ground instead of into the drainfield.
The tank is full of solids, which are slowly migrating and may eventually clog the drainfield. This may increase the cost of pumping the tank and may require replacing the entire drainfield if it becomes clogged.