System flushes should be carried out on installation and if you move to a new property and you're unsure if inhibitors have been added, it's worth fluishing and adding inhibitors. it will save you money.
If inhibitors are added they will not need re-plenishing - only if the system is drained.
There's no timescale to say when a system should be flushed, if you notice sludge when draining the system. Cold spots on radiators, poor circulation or failure of parts beacuse of poor cirulation or a build up of limescale or sludge - a flush may help.