> The recommendation from the installer is that it is pumped and inspected every 3-4 years.
That sounds about right if the goal is to keep a system that's not properly functioning (not enough microbes to break down solids) from filling up to the point of causing problems. You won't go wrong if you follow that advice but depending on what goes into your septic (do you have a garbage disposal, do you do laundry with bleach, number of people in household, etc) you may be able to go an order of magnitude longer. Recommending a system be pumped every 3-4yr is like a 3k oil change, it's excessive in the overwhelming majority of use cases but it will make more money for whoever does the work and not cause a system to fail in a way that comes back to bite whoever installed it.
That sounds about right if the goal is to keep a system that's not properly functioning (not enough microbes to break down solids) from filling up to the point of causing problems. You won't go wrong if you follow that advice but depending on what goes into your septic (do you have a garbage disposal, do you do laundry with bleach, number of people in household, etc) you may be able to go an order of magnitude longer. Recommending a system be pumped every 3-4yr is like a 3k oil change, it's excessive in the overwhelming majority of use cases but it will make more money for whoever does the work and not cause a system to fail in a way that comes back to bite whoever installed it.