I have many different gages with calibration ranging from 6-mo to 5-yrs, (and my ISO-9001 CB auditors have never written a "non-conformance" nor "observation"). These intervals are based on historical use and calibrations. Many of my 3-yr gages, (max interval for production gages), still have the wax from the last calibration when they get sent off... why, because the interval says 3-yr. We do not have any 'if not used, extend' allowances. My set plugs are on a 5-yr plan as they are used at most twice a year, depending on ring gage usage, and then only by me.
I send gages to our outside cal lab and specify interval for each gage. They don't complain, just note in my records "customer specifications". The same is true for our gages that we have modified from "industry standard" on tolerances; the cal lab makes note of the deviation and they calibrate to that. Our QA Manager has to buy-off on all deviations from manufacturer/industry standards, and then, after a risk assessment and agreement with the appropriate production manager(s) and Factory Director (we have a 3.5"-6-UN plug gage that they allow +.0030", since part tolerance is +.0052", although industry standard says +.0004"). Any change I want to make to intervals, needs to be approved by the QA manager after presenting my justification/Risk-assessment to him.
From an ISO view - ISO does not care too much about what the procedures say as long as that is what you do. If it says to soak your gages in the calibration room 48-hours before calibrating, then they had better soak for 48-hours. Customer and the industry might frown upon your "extension", but ISO doesn't as long as it is part of your written procedure and you can justify your reasoning and the Risk to your product (as an auditor, I will ask for this).