It is good you live in the country and you can can get away with it. If you live in MA it gets reported to the Board of Health and they will tell you you have to replace the field and indeed you are looking more at $30k.
Since what you did is illegal here, people usually have their tank pumped every 3 years depending on the size of the tank. I know people with small tanks that do it every year.
We know where our tank and distribution box are. Also houses can not be sold here without a septic inspection.