As far as I know this has not yet been determined. My own view is that any measurement, no matter how old, should have immortality as long as it has been under the observation of the original measurer.
I recognize that this can pose administrative problems.
I have a calibration course on my street that is 20 years old. I recently remeasured it and recertified it, but considered this to be overkill, as nothing had changed. Repaving had occurred, but the end point landmarks (curb joints) were not affected.
I believe the key to this is the personal oversight of the original measurer. No delegating.
I recognize that I am not the RRTC Chairman, nor officer, and that I consider my views to be individual opinions only. I am perfectly willing to go along with whatever may be decided on the subject.