Interesting question - many variables. Assuming that oxidization (rust) would be the biggest factor in determining if the shelf life was jeopardized. One would think the structural (flex) of string should remain constant as long as it doesn't rust.
In an ideal situation the individual strings sealed properly with an inert (nitrogen type) gas should last forever. No oxygen contact, controlled humidity level when packaged and no oxidization should occur.
If a string was packaged and sealed with moisture inside then degradation could occur.
Paper sleeves/ cardboard could be bad OR could act as a moisture wick and slow/ reduce oxidization from occurring. Some papers used for wrapping metal (not sure if they are used for strings) are treated with an anti-oxidization substance. It slows the oxidization process and would be a great thing for strings.
Then there is the consideration if any of the strings were dipped/sprayed with an anti-oxidization treatment (oil/ grease/ etc).
Again I would ass-u-me that string manufacturers probably have some type of 'slow the rust down' contingency plan as they make the string, package it, it sits around waiting for shipment, it gets shipped, sits around waiting for final delivery, gets delivered and sits in inventory for who knows how long awaiting for someone to purchase it . . . .
So with that in mind ultimately it would depend on different manufacturer's and their SOP (and execution of SOP during their process). So you could/ should/ would (?) see a variable rate of shelf-life from package to package.
I guess I really don't know...
Paul (who hasn't had much sleep the last few nights and finds himself over-thinking things)
(and sometimes sounds like he could be a politician by saying an awful lot of things but not really making a point)