It can keep flushing that whole time, which is exactly why people do it. What happens instead is that solids carry over into the drain field and destroy it, turning a few hundred dollars of pumping into an $8,000 to $15,000 replacement.
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The reason this question gets asked is that skipping pumping produces no immediate consequence. Toilets keep flushing, drains keep draining, and nothing announces a problem. The damage happens somewhere you cannot see. Here is the mechanism. A tank works by letting solids settle to the bottom as sludge and grease float to the top as scum, leaving relatively clear liquid in the middle to flow out to the drain field. As the sludge layer builds, that clear middle zone shrinks. Eventually solids reach the outlet and travel into the field, where they clog the soil pores the whole system depends on. Soil that has been sealed by carried-over solids does not recover. The field is a consumable that was destroyed early. The arithmetic, which is the honest argument. Pumping runs a few hundred dollars every three to five years, so on the recommended cadence it costs on the order of $80 to $130 per year. A drain field replacement runs $5,000 to $12,000, and a full system replacement $8,000 to $15,000. The EPA SepticSmart three-to-five-year interval is deliberately conservative because the asymmetry is so large that being early costs almost nothing and being late costs everything. What moves the interval. Household size and water use. A garbage disposal, which adds roughly 50 percent more solids and pulls the cadence to every two to three years. Tank size relative to the household. The good news for anyone who has let it slide. A tank pumped late is usually fine. The field is what suffers, and if the drains still run normally and nothing is surfacing outside, the field has probably survived. Get it pumped, ask the pumper to check the baffles and record the sludge depth while the lid is off, and reset the interval from that measurement rather than guessing. Central Ohio sits on flat-lying glacial till with dense clay subsoils that drain slowly and perch water above the till rather than letting it through. Local soils are why the field is the part worth protecting here.