If you let the fat and oil solidify at room temp and dispose of them in the trash, and if you use regular soap in the recommended amount, and if you septic tank additive once in a while, you will be tearing the problem at the source, and will only need to take care of the tank when it’s time to replace it in 40 years.
Septic tanks are not closed systems they are open and intended to provide an environment that can produce a bio-reactor while maintaining drainage through the soil.
Many good systems don’t need to be drained other than every few decades in fact good systems would outlast the tank and the filtering substrate it’s built on so when the time comes you need to dig out the tank and the gravel and other filtering particulate it used because that is what has degraded.
The problem with septic tanks is that they don’t scale we’ll have one on a large property in the middle no where that 5 people use and it’s a great solution have a bunch of them and worse over use one in particular and you get a polluting mess.
But septic tanks also don’t solve the other problem with western toilets and that is the amount of water they consume which is simply unfeasible for many developing regions.
TLDR; if the septic tank was installed and sized correctly it will outlast you even without pumping it, but it’s not a solution the reason we don’t use them in cities is because they don’t and can’t scale with urban population densities.
Other areas like those in which ground temperatures drop below freezing for long period of time might also require tight tanks and constant pumping since the cold may stop the reaction in the tank so it would surprise me if tanks in places like say N. Dakota would be pumped year before winter to prevent them from freezing shut and overflowing.
Every few decades for a properly sized and properly functioning system.
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.
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.