The unions may announce they have filed a strike motion weeks in advance (partially as a way of pressuring the government to negotiate in the interim). I think the absolute latest they can file a strike motion is five days before they plan to strike.
By law, the union workers who plan to actually strike have to notify their management by 48 hours ahead of the planned strike.
At that point, the SNCF (French rail company) takes that info (the declarations by each employee who plans to strike) assesses it, and creates its alternative minimum service schedule, which is usually released the evening (about 5 pm) before the strike.
The hard thing to guess at is : some strikes end up being a huge deal, and some totally fizzle. And of course it's impossible to know ahead of time. (People who have been around a while watching these things can make educated guesses, but of course that is not an advantage afforded the visitor.) So - just because a strike has been announced doesn't mean it will be problematic. On the other hand, it definitely CAN be!!!