Home built 1996. Has had 4 people living in it since then. We do not have a garbage disposal. We have washer/dryer, dishwasher use about twice a week. The septic tank has never been pumped. We were not aware that this was something we needed to do but recently our neighbor had it done after 15 years himself so we thought we should check into it.

Mostly the time is going to vary by many factors. Really the only time would be if it fills beyond capacity. Also depending on how it is set up - easy access or have to dig it up. Will also be a factor. Local water tables will also have an effect on it.

If your not having problems then don't worry about it.