I've used this product over four years now and just had my heat Exchanger replaced in my Worcester Combi 30Si under the guarantee for the boiler. The boiler engineer suggested that this SESI Salamader had failed after four years. The Combi heat exchanger was getting clogged up with scale. We do live in a high scale area in Gloucestershire.
I would therefore suggest you replace this product every four years if you live in a high scale region. If not then who knows. The first sign of a problem with a scale inhibitor is when your hot water taps deliver reduced pressure flow. This should be noticeable. The radiators should be working normally. The hot water taps will not.
What is happening is that your heat exchanger in the boiler is gradually getting clogged up with scale. It will soon have to be replaced. You better hope your boiler is still under guarantee. If not then you will have to pay for a new heat exchanger and the cost of fitting. A very tidy sum that will be.