IMO, if you are the one defining the intended use of the device (ie, if you are the one marketing the device and putting your name on it), you do need to identify and document the use life yourself. You need to consider the intended purpose of the device and what "real world" use would look like. However, the use life is going to be dependent on the design, so you probably don't actually have much control over what it is. Basically, the extent that you "define" the use life, and what you do with it, is commensurate with the activities that you perform.
Presumably you had some level of use evaluation before purchasing the product, based on your intended purpose. For example, if your device is a reusable metal surgical tray, and your supplier says that the use life is 1 week, that would not be acceptable, right? So, even if you are not making design decisions to meet that use life, you are still "defining" a level of acceptable performance. The things that you are doing to meet that requirement are just different (ie, you are selecting a supplier/device that will meet your use life - you are not choosing the material).
Another example, if your device is a surgical drill/handpiece - if your supplier defines the device use as 1/x week for 2 minutes each time, but your use case is 5x/week for 1 hour each time, the difference in the intended use will likely result in a different use life. So, even if the supplier does "define" a use life, you need to make sure that their assumptions/inputs are the same as your marketed use.
The lifetime needs to be defined and properly reflected throughout the rest of the product documentation as well. For example, if your device has a lifetime of 5 years, have you performed reliability testing that demonstrates that use life? Does your risk analysis properly analyze the risks of using a device that is too old? Do you communicate to the user in the IFU when the device is no longer functional? Does your Post-Market Surveillance system take this into account? (eg, if you get a complaint for a device that is 7 years old, do you consider the device age in the analysis?) If you do servicing or repair, how does that factor in? (eg, does the "use life" start over after the device is repaired?)
IMO this doesn't necessarily need to be a huge endeavor - you just need to show that you've considered the use life to the extent that it impacts the activities you're performing.