It depends.
The short answer is "24 hrs of validity from compostage, if you are lucky"
If your ticket includes a reservation for a mandatory reservation train (i.e., TGV, Corail Lunéa, Corail Téoz, international services...), then you must use it only on the train specified on the reservation -- it is not valid on any other train on the route, regardless of whether the other train has mandatory reservations.
Obviously, you cannot use a mandatory reservation train without reservation, however many ticket types allow you to exchange up to the time of departure, and there are indeed machines on many station concourses that can issue exchanges, if you don't fancy queuing for the guichet.
If you are on non-reservation mandatory services, with an unreserved ticket, then the ticket is valid from:
- 24 hrs from the time of Compostage (stamping), or
- 24 hrs from the time of your first train, in the case of an e-ticket or print-at-home ticket.
If your break of journey pushes you from one fare calendar colour into another (i.e., pushes you from off-peak to peak), you'll need to have a peak (white) fare ticket, as the only occasion that this is not required is when you change onto the next available train (i.e., don't break your journey)
And I should mention that aller-retour (return tickets) explicity state that you must use the same route on your return portion. It is possible to tell, because you pay per km, so the price will differ. Circular tickets exist for some flows, but you have to nominate a start / end station when you buy them.
I believe tickets in Île-de-France (Greater Paris) have separate conditions, as ticket prices, validities, etc. are determined by STIF rather than SNCF. You may also find that some other cities have separate arrangements for local trains, particularly if the city in question is running a tram-train service in place of traditional services.
Finally, I should note that this is based on my own personal translation of French legalese. You should check with a guichet.