I seem to remember (5 years ago, or so) I had some metro tickets, which two years later did not work--maybe the magnetic strip lost its magnetism
, sitting in my wallet for two years.
Heh... I was going to say in my original post that about the only reason they might not work is that they've been demagnetized...
While buying tickets last week, I overheard an RATP agent reassure a gentleman that the stash old purple tickets in his wallet hadn't expired.
I've occasionally bought a Mobilis and held onto it for some time before using it, though not as long as the OP envisions. There, too, you'd have to take care to not demagnetize the card or the convenience of the advance purchase would be nullified.