Most domestic systems need a power flush roughly every five to eight years, but the calendar is the least reliable guide. Condition is what matters.
A sealed system with a magnetic filter that gets emptied at every annual service can go a decade or more without needing another flush, because the filter is catching the corrosion products as they form. An older open-vented system with a header tank in the loft draws in fresh oxygenated water every time it tops up, which keeps the corrosion going, and those systems silt up noticeably faster.
The symptoms arrive before the anniversary does. Radiators going cold at the bottom, the house taking longer to warm through than it used to, kettling or banging from the boiler, or the water coming out dark when you bleed a radiator — any of those means it is time, regardless of when the last flush was. If none of them are happening and the filter is coming out reasonably clean each year, the system is telling you it does not need one yet, and we are not going to sell you a flush you do not need.