The
RATP site says "After validating it every time he boards a bus or tram, the customer has an hour and a half between the first validation and the last validation to make one or more connections."
I did try to reuse one a couple of hours after first use (2 hops, left system, tried to re-enter), but it was rejected. I assumed the policy mentioned above is in force, but perhaps someone with more experience will jump in here?
Edit: after further reflection it would seem the 90 minute expiration wasn't the reason I couldn't re-enter on the same ticket. the assumption mentioned in this post seems to be invalid, now that I think about.