First, ask yourself the following questions:
If your answer to any of the above questions was "no", then you already understand the importance of having your gas boiler serviced routinely.
Getting your boiler serviced helps you avoid all of the above problems, and probably quite a few others that aren't mentioned here. It's an absolutely essential task, especially if your boiler is starting to get on a bit in years.
What are the main reasons to get my boiler serviced?
Safety is the primary issue, and you should never underestimate the potential safety risks associated with gas and other combustibles. Observing the proper precautions and only allowing licensed professionals to perform any work on your appliances can significantly cut down the danger.
Efficiency should be your other concern. If your boiler is not properly maintained, it may not be operating as efficiently as it should be, and that can add quite a lot to what you're paying for utilities.
When should I get my boiler serviced? How often?
As a general rule you should get your boiler serviced:
It doesn't do any harm to have your boiler serviced pre-emptively, even if it turns out there's nothing wrong with it. On the other hand, not having your boiler serviced when it needs attention could cost you a lot of money, and even more importantly it could cost your life.
How long does the service take to complete?
There's no simple answer to that question because it depends firstly on the individual contractor and secondly on the condition of the boiler. If no faults are found and everything is fine, it could take less than an hour to complete the job.
Who should service my boiler?
Definitely this is not the sort of job to tackle on a DIY basis. You need to make sure you get a licensed professional with training and experience in maintaining gas boilers. Otherwise you run the risk of substantially higher costs and potential damage or danger.
While it may seem a thrifty option to hire unlicensed workers, there's probably a reason why they're unlicensed. That being the case, they may not have the right tools for the job, they may not have completed their training, or they may have engaged in some misconduct that led to the loss of their license.
In that sort of situation, it is definitely a false economy to hire them because any mistakes they make will lead to more costly repairs being necessary down the track, and of course there's the possibility that if they don't do their job correctly, real harm could result.