The honest range is 20–30 years — and almost everything that decides which end you get is under the owner's control.
A leach field fails when the soil around it clogs — a biological mat builds at the soil interface until water can't infiltrate faster than it arrives. That process is inevitable, but its speed is set by what the field receives. A field fed only clarified liquid from a well-pumped tank clogs slowly; a field receiving solids from a never-pumped tank clogs in a fraction of its design life. The 20-vs-30 question is mostly answered by the pumping schedule.
Pump on schedule (the whole game), fix running fixtures fast, spread laundry across the week, keep everything heavier than a lawnmower off the field, keep trees back, and keep grease in the trash. None of it costs anything beyond the pumping.
Fields rarely die overnight. The progression: drains slow seasonally (wet springs first), then persistently; soggy or lush green stripes appear over the lines; odor follows; finally water surfaces or backs up. The confirming test happens at a pumping — if the tank refills from the field side after being emptied, the field is rejecting water and the conversation changes from maintenance to planning. Even then, planned replacement on your schedule beats emergency replacement on the system's schedule — and partial measures (resting a section, adding capacity) are sometimes possible when caught early.
Typically 20–30 years. Pumping history is the biggest variable — fields fed solids from neglected tanks can fail in half their design life, while well-maintained fields regularly reach 30+.
Sometimes, if caught early — resting a section, repairing a distribution box that overloaded one trench, or adding capacity can buy years. A fully saturated field, confirmed by water rejecting back into the tank, needs replacement.
Mechanical aeration and similar treatments exist and occasionally help marginal fields, but results are inconsistent — be wary of anyone guaranteeing revival. No additive poured down a drain restores a saturated field.
Check the town office for the NHDES design on file — permitted systems show installation dates. No records plus an older home means the pumping-visit evaluation is your best estimate of remaining life.
Sludge levels, baffle condition, and field response at a pumping tell the story — get the honest read.