Once a year.
And I am telling you now because August & September are the best months to have it done.
Any time of year would be OK but late summer/ early autumn seems the most popular time and this seems most sensible.
Get it checked out before winter, of course, perhaps identify any problems when the weather is still mild.
Who says once a year? Well, check the owner's manual. It will almost certainly say an annual service is recommended.
If the boiler is new-ish and still in the warranty period then the manufacturer require a boiler service within 12 months or the warranty becomes invalid.