From a firm that only sells radiators, etc (and therefore ought to know a thing or two about them) but doesn't offer plumbing services (and therefore has no vested interest in getting customers to have the job done at very frequent intervals):
"If you want to keep your system in tip-top shape then power flushing every 5 to 6 years is a good idea.
Still, you should be able to get away with power-flushing your radiators every 8 to 10 years depending on the quality of the materials used in your system."
Source:
https://www.onlyradiators.co.uk/blog/heating-essentials/how-often-should-you-power-flush-central-heating
As a read that, the plumber isn't telling porkies just to get some extra work but the job's not actually essential at the moment.