From today, the standard validity of TER tickets changes from 61 days to 7 days. See
LES TARIFS VOYAGEURS: CONDITIONS G^RALES DE VENTE SNCF▸ (just the 216 double-width pages of it - several sets of regulations plus a national fares list). This was presented in the news as an anti-fraud measure.
This limit does not apply to trains with compulsory reservations, for which tickets which will be dated (all
TGVs▸ and some Intercit^s), and the 7-day limit was already in force on Paris suburban trains (R^gion Ile-de-France). This 7-day limit is the validity from the ticket date, and interacts with a separate pre-purchase limit of three months, i.e. from purchase to use. If you have used one of the standard station machines on which you have to make interminable selections by spinning a wheel, you may recall being offered a choice of 61-day validity periods (about ten to cover a month, I think). That now has to change, so I guess the machine will have to offer even more choices, or even all 60 future 7-day validity periods.
Incidentally, I also looked in those regulations for break of journey. There is a general requirement to complete the whole journey within 24 hours, in excess of which a new ticket is required. Outside Paris barriers are rare, so a break within a day is no problem. International trains are not covered by these regulations at all, so maybe even the French internal part of a journey on classic old-fashioned cross-country trains would not exceed 24 hours. Oddly, for a rather legalistic document, I can't find anything to cover the impact of delayed train running on this. There are provisions for delays of over an hour, including a right to go back home or to continue within 24 hours (Annexe 7).