"Usual" sales opening
To complement other answers, indeed the sales start exactly 90 days in advance on most trains - TGV and Intercités as @audionuma quoted. Be careful though, some trains like regional ones have specific fare policies that include different starting dates, and that depends on each region (e.g. I tried on the PACA TER website and I could add a ticket to the cart for a trip in 1 year). The price and availability does not vary on these regional trains so no need to book in advance though.
However, some products sometimes have different policies on the starting date, in particular iDTGV, Ouigo, TGVs and Intercités.
Unusual: TGVs and Intercités for school holidays
For the TGVs and Intercités, there are three periods of the year when tickets may be sold earlier: Christmas, winter holidays (usually in February) and the summer. If you have a SNCF fidelity card or are a member of Capitaine Train, you shall receive emails warning you in advance of the sales opening for these popular periods.
Here I quote an email received on March 28th that sales for the summer (until August 30th, the last sunday of the summer holiday period) opened on April 2nd:
Pour vous remercier de votre fidélité, nous avons le plaisir de vous informer en avant-première de la prochaine ouverture des ventes de billets de train pour voyager cet été !
Tenez-vous prêt(e) ! Le 2 avril dès 6h, TGV et INTERCITÉS ouvrent les ventes pour vos voyages jusqu'au 30 août 2015 en France et quelques destinations en Europe !
I received similar emails for other periods, so in the end the advance booking was available as follows:
- on November 7th 2014 sales opened until March 9th 2015
- on April 2nd 2015 sales opened until August 30th
- on October 16th 2014 sales opened from December 14th to February 6th
These are indicative dates, if you plan on travelling for popular periods and want to take advantage of low prices, you should expect to book around these dates in future years.
Unusual: iDTGV and Ouigo
For Ouigo, a service intended for low-cost travel, and for iDTGV, another TGV service, the dates are different:
- on October 1st, 2014, sales opened for Ouigo until July 6th, 2015
- on September 16th, sales opened for iDTGV until March 29th, 2015
So in the end while the common rule is that sales are opened about 3 months in advance, this varies a lot depending on what train you want to take. If you want to travel during school holidays in particular, the sales opening may be earlier (or later) than the usual three months. If you want to be notified by email of these sales openings, you can signup for an account at voyages-sncf or Capitaine Train.