It is a common assumption that service life should be derived from the properties and testing of the actual medical device. This view is even supported by ISO TR 14969 (guidance on ISO 13485), which states in Clause 7.1.3 that the "... basis of the defined lifetime of the medical device should be documented" and goes on to suggest items to consider.
Fortuntely this view is wrong, and is an example of the blinkered view that can sometimes occur from different medical fields. For some simple medical devices, it is feasible to consider lifetime as an output of the design process, or the result of consideration of various factors. But that's far from true for complex electronic medical devices such as those often covered by IEC 60601-1.
The correct interpretation (regardless of the type of medical device), is that lifetime is simply something which is decided by the manufacturer, and there is no regulatory requirement to document the basis of the number chosen.
It is a requirement that the lifetime must be declared and documented. IEC 60601-1 Clause 4.4 simply asks that this is stated in risk management file.
And, having declared this lifetime, the manufacturer must then go on to show that risks are acceptable over the life of the device.
For some medical devices, lifetime will be an important factor in many risk related decisions, such as sterility, mechanical wear and tear and materials which degrade over time.
For other medical devices, lifetime hardly gets a thought in the individual risk controls.
Why?
For electrical devices we are a little different in our approach. These days, modern electrical parts last for much (much) longer than the lifetime of the product. And there are thousands of parts in a single product. Inevitably there will be the odd part here and there that breaks down earlier than others, but on a component basis it very rare and hard to predict.
Secondly, we rarely entrust high risk stuff to a single part. We assume that things fail from time to time, and implement protection systems to prevent any serious harm.
There can be cases where lifetime does play a role, but it is the exception rather than the rule. Even then, it would be rare that the lifetime of a part or risk control drives the overall decision on the medical device lifetime. Us electrical engineers don't push things to the edge like that. The risk management might determine that a particular critical part needs a failure rate of less than 1 in 10,000 over the 5 year lifetime of the device. So, we pick a part with 1 in 1,000,000 in 10 years. It's just a different way of thinking in electronic design.
So the next time an auditor asks you how you derived the lifetime of your incredibly complex X-ray machine based on as risk, quietly direct them the marketing department.