Roughly every 1 to 3 years for most households — the exact interval depends on the tank's size and how many people are actually using the system, not a fixed calendar date.
What actually decides the interval
A small tank serving a full house works through its capacity faster than a large tank serving one or two people. Water-heavy habits — long showers, washing machines and dishwashers running often — also shorten the gap between empties. If you don't know your tank's size or when it was last done, that's the first thing worth finding out rather than guessing.
Warning signs it's overdue
Slow-draining sinks or baths across the whole house (not just one), gurgling drains, unusually lush or soggy grass over the drainage field, or a bad smell outside near the tank — any of these mean it's worth checking sooner rather than waiting for the "usual" interval to come round.
Why leaving it too long costs more
Once solid waste builds up past the tank's capacity, it starts reaching the drainage field itself. A clogged drainage field is a significantly bigger and more expensive job than a routine empty — this is the main reason to stay ahead of it rather than wait for a visible problem.
When to call
Not sure when yours was last emptied, or noticing any of the warning signs above? Call 07468 868807 and we'll talk through what you're seeing and get it sorted before it becomes a bigger job.