If you ask any Rolex or Omega or other high-end watch repair shop, they'll gladly remind you that all the critical rubber seals age and fail, so that none of the makers will guarantee them to work after just ONE YEAR.
That would imply the same rubber O-rings and seals used in "intrinsically safe" radios should also be changed annually, unless the maker specifies a longer period. There's no universal answer, it will depend on construction techniques. If the guts of the radio are hermetically sealed, the switches all magnetic contacts or polymer separated, and the case has been welded shut (which is common in marine electronics, to avoid failing gaskets) then the radio could be safe for many years.
You'd have to know the specifics of any one radio to rely on the answer. Or, just ask a friend to take it into a methane atmosphere. If they come back out again, under their own power, then it probably is still safe.(G)