Default to twenty years in the "real world", Mammad. As long as equipment is (or has been) properly maintained, that is. As I keep saying, what really matters is the
condition of equipment, not how long it has been on the inventory (or, for that matter, sitting on the shelf, or in the cupboard). Properly conducted preventive maintenance remains the name of the game, my friend.
However, if you're trying to "make a case" for getting rid of a pile of junk, you might like to tell "them" that spare parts are no longer available much beyond ten years, or something like that. But, as I have indicated, why not just tell them it's a pile of junk?
Remember too that there may be many reasons why equipment should be withdrawn from service (uneconomic to continue repairs, clinically obsolete, and all the rest). Sometimes, even perfectly serviceable kit just has to be "binned" (regrettably).
Lastly, in some parts of the world, manufacturers (or their agents) have a legal obligation to support equipment (
eg, by having spare parts available) for a certain period of time (often as little as seven years only). In my opinion this is a disgrace (which can be got around by thoughtful stocking of spare parts
in-house)!
