To maintain and prolong the life of your boiler it is recommended that you have it checked every 12 months to allow an engineer to inspect and verify the state of the boiler and it's safe operation. A typical modern condensing boiler will require a full strip service every 3-5 yrs depending on the manufacturers instructions. This is to allow the heat exchanger to be cleaned of any debris that may be contributing to poor combustion. Burner seals and 'O' rings will also need to be replaced.
One of the signs that your boiler needs a full strip service would be the results of a gas analysis test of the gas/air ratio at the flue testing point. A gas engineer will perform this task at every service as a matter of course to determine whether your boiler needs an annual service or a full strip service.
Please make a note of the make and model of your boiler when considering booking any service. If a full strip service is required then the engineer would need to have the correct seals with him on the day.
A typical annual service would typically last approx 2hrs. A strip service involves deconstructing the boiler to gain access to the burner and heat exchanger and therefore is a lot longer and more expensive (Labour & Parts required)