In my experience, in this context the life-time only becomes an issue when deciding how long documents need to be retained for. In general MTBF type testing should not be required. The lifetime is really a justification for your record retention times and I would base your rationale for retention around lifetime. I would add the lifetime into a suitable procedure, such as document control. There's no requirment to put it into an IFU unless there are safety of commercial reasons.
Also, have a read through ISO14696, Section 7.1.3, which says that the ...lifetime may be based on technical, legal, commercial or other considerations...", and then expands on it further listing numerous items to consider. It also says the rationale should be documented.
One last observation, in the past I have been written up in a 13485 audit for not having the lifetime defined anywhere!