A typical domestic power flush takes four to eight hours, so most jobs are finished within a single working day and the heating is back on the same evening.
The time goes on the radiators. The pump gets connected to the system, then each radiator is isolated and flushed in turn, with the flow reversed and the panel agitated to shift the sludge that has settled at the bottom. A six-radiator flat can be done in a morning. A ten-radiator house is usually a full day. Where the system is very heavily silted, or the pipework is old small-bore that clogs easily, it occasionally takes a second visit to get the water properly clear — we would tell you that was likely at the survey rather than springing it on you halfway through.
We finish by refilling with clean water, dosing the system with inhibitor, and checking the pressure and the balance before we leave. You should notice the difference the same evening.