I'll throw my response in, because this is always a topic on my mind...
Our house was built in 1937 and still has the original BRICK septic tank for it's "conventional" type septic system. The d-box was replaced at some point with a masonry block construction box (which I don't love) and the field was updated in the 1990's with a 410-ft long field line that just wiggles through the grade of our back lawn. We bought the house in 2012 and had the tank pumped and repaired (mortar). The repairs were done while we were gone so I didn't see the tank when they dug up the lids.
We haven't pumped it since then (13-1/2 years) and we're really careful about what goes down the drain. None of the fancy additives, definitely no paper anything except toilet paper, there are 4 of us in the house (including 2 two kids!), we avoid putting anymore solids down the drain, but if food IS going down the drain, it's minimal.
All that being said... I'm replacing and moving the tank, at a minimum. It's too close to the house, I don't trust the age or construction, and it's in the way of future home renovations and I want to configure the inlet line and location to work with my future shop build, since it will have a bathroom.
Rumored quotes for my area of North Carolina are 6-10K for a conventional system installation with field. But I'm still trying to find someone that will come out an quote it. People don't want to talk about septic systems unless you already have the permit pulled, which seems completely back-assward to me. So it's been tough to get anyone out. Either way, I'll have to do this, soon. I'm nervous.