You can expect a conventional septic system, such as that being described here, to last about 30 years. Some systems last much longer and some systems can fail earlier for reasons like those noted above. Other things can also affect the life of a septic system. For example, a system may have been providing satisfactory service for a previous owner for many years, only to fail shortly after you have bought the house. If the previous owners were a working couple with no children, the system was probably not heavily used; if yours is a family of six, the added load could push a marginal system over the edge and into failure.