For FT use by a couple, roughly every 6 weeks or so and it can be stretched to 8 weeks (considerably longer if you're not using it every day, or give it a break when you're in a marina or away from the boat). I have friends who do it every four weeks or so but only change out half the old peat to freshen it up. It is honestly not a bad job at all, and feels (and smells) more like working in the garden than anything (The exception is if you get too much liquid in the peat, which has only happened once and I'm not sure exactly what happened).