A properly designed tank has two chambers (or, old ones have two tanks). First chamber in-line from the residence is the "solids" chamber. Then a baffle (actually two, one from the ceiling and one from the floor) and chamber two is the "liquid" side, which is a bit of misnomer. The liquid side is still a digester and it is working on colloidal suspensions of minute solids. The baffles are to keep heavy solids from migrating along the floor and floaties from getting into chamber two along the surface. Only neutral buoyancy liquids should go from chamber one to two...
Things like egg shells, meat bones, some veggie waste will not digest in a reasonable amount of time and slowly accumulate in chamber one. The general recommendation is to have chamber one pumped every 5 years, but that is usually over-kill. I have seen tanks with laundry and appliances including garbage disposals go ten years before the solids start to tip over into the liquid side ...
10 years is a good number for general pump-out. But having it done at 4 years will tell you a lot. If the solids side is less than 1/2 full (1/2 way up the lower baffle), you are probably good for the 10 year number... If it's more than 1/2 full, you are better to schedule at say 7 years.
Flushing after-market enzymes down the toilet is OK, but it's a lot like MMO in an engine. It's mostly insurance. And you never know from where or what breed of enzymes they are selling you. 99.9% of all the bacteria and enzymes you need are already in the waste stream. It's a pretty unusual situation to really need add treatment.
If you are willing to allow your tank to become solids digesters on both sides, you can get up to 30 years on one pump. But you put your leach field in jeopardy doing that. The leach field should only be handling clarified waste water, never any solids (even colloidal minutia) ...