At my place, it's just my wife and I. We had not had the septic cleaned since we bought the place 18 years, ago. I had it done 5 months ago for the first time. We have always been careful not to put vegetable oil, colored toilet paper, etc. down the drain.
I removed the soil on top of the tank myself, just to check things out. I found two clean out doors on top. Then, I called the septic truck. It was interesting and fun to watch. The clean out guy used a pick axe to pry the door tops off which are designed to wedge themselves in the tank top. Then, he got a long metal rod and stirred all the stew in the pot. It had a 6 inch crust on top. It smelled, but I have smelled worse things. Then he put the suck hose in the tank and pumped the liquid in and out of the tank several times while stirring the hose end to loosen all the solids in the bottom. Finally, he sucked it nearly dry. All the time he was doing this, he was smoking cigarettes and even ate a sandwich between sucking and spewing. I kept watching where he put his hands. He did not bother washing his hands or anything. One hand would hold the cigarette while the other would hold the hose. How come I get the runs over nothing...when that skinny septic guy smokes and wipes his eyes and mouth with sewer hands and not be sick?
Anyways...he said that our septic really was not bad, really did not need to be cleaned. He said to keep doing what we were doing to keep it going good in the years to come.