500ml is suited for a system of 10 radiators or a 100 litre system, so you shouldn't need more than a bottle unless you have a larger system than that. The guide is that your plumber checks the inhibitor concentration when your boiler is serviced annually, and as long as the concentration levels are at the recommended level (10%), it would never need to be changed. If the concentration has dropped, then you would merely top up the inhibitor to meet the concentration level. Plumbers have a TDS Meter to check these levels, so they would know the level it would need to be at, but generally, once the inhibitor is added, it shouldn't need to ever be changed, only topped up if the level is low.