When I was a young boy, my dad said a properly installed and treated septic tank should never need pumped. He lived on that farm for 40 years and never had an issue. I think Trigger Time is right about today's household chemicals being harder on septics. However, I have lived at my current home for 31 years now. Thought we had a full tank about ten years ago, but it turned out to be a clog where the pipe goes into it from the house. Had a company come out and they found and cleared the clog. They said there was about three feet of clear space in the tank, but I had them pump it out anyway. Haven't had anything done since, but it is on the list to have it pumped out again. BTW, 4 bedroom, 2 bathroom on 1/2 acre.
Several of my neighbors and two excavating contractors have told me that our 15 year old septic system is one of the few in the area that is not is some way tied into field or county tiles that eventually drain to old man made drainage ditches. Over the years many folks have diverted their septic drainage when issues have came up unbeknownst to others . Quick fix... I don't think all the four and five decade old systems are able to meet modern code.This. If you have to have your tank pumped every 12-24 months, something is wrong. My boyhood home was built in 1967. Its tank has been pumped twice in that time frame. Works fine. Without so much as a box of rid-x.
Then again there is no automatic dishwasher, garbage disposal, no "flushable wipes", etc.
Several of my neighbors and two excavating contractors have told me that our 15 year old septic system is one of the few in the area that is not is some way tied into field or county tiles that eventually drain to old man made drainage ditches...
What it comes down to is consistency and contents. What goes in should only be what goes through the body already and water. Some soap's OK, as others have said, anti-bacterial is a hard no. After that, consistency. If you like going on long vacations or having a house full of visitors it'll hurt the system.
The whole premise is that it's self-sustaining. Think of it like a living thing cause it is. The tank shouldn't be emptied and filled, it should stay mostly full and the bacteria eat the solids and the liquids drain to the leech field. If the field stinks, something's wrong. If the tank's "full" something's wrong (what you're feeding it, built wrong, or broken). You shouldn't have to pump it every year or even every 5.