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The range is wide because it depends on tank size, household, and whether you run a garbage disposal, which loads the tank far more than people realize. For three people on a 1000 gallon tank, every three to five years is the usual landing spot. The honest answer is that an inspection tells you more than a calendar does, since they can measure the sludge and scum layers. We get ours done through Rock Solid, and their Septic Services side does the inspection along with the pumping so you’re not paying twice. Also, our clay soil means a struggling field shows up late, so I’d err toward the shorter interval. If you do have a disposal, cutting back on how much goes down it buys you real time between pump-outs.