> 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.

The manual for the original Nintendo Entertainment System said "Do not operate continuously for more than 15 minutes."

Sometimes legal departments force them to include those recommendations in order to err on the side of "absurdly overcautious." And sometimes the recommendations are merely prudent! I'm not saying you can wait longer, just that installers may have a motivation to exaggerate.

But unlike an NES system, if your septic tank develops a problem you now have liquid and solid waste flooding your yard and home, and your toilets, sinks, and showers are useless until it's fixed.

I feel like the failure mode for electronics is "randomly catches on fire" which can be just as devastating, if not moreso. Nobody has ever died because their toilet backed up. Their house burning down in the middle of the night is a different story.

People die from diseases caused by sewage, but it's usually a downstream issue.

My grandparents had there's installed in 1983. I think it was a somewhere between 700-1000 gallons. The toilets finally stopped flushing properly in 2010. It was because the tank was completely full of sludge. They got it pumped, and I guess it'll go another 20+ years now...