Personally, I don't there is really a latest possible partition of France to be honest. But that largely depends on which polities you would want to come out of a partitioned France, and also if you have enough military might to enforce a paritition or enough covert support to any one of the factions aiming to split off a particular province from France. (At the time of this writing there are seven active separatist movements (French Basque Country, Brittany, French Catalonia-Roussillon, Corsica, Savoy, Provence, Occitania) and seven active autonomist movements (Alsace, Brittany, Corsica, Nice, Normandy, Savoy & Occitania) within Metropolitan France.) Granted, it is highly unlikely (barring perhaps whatever happens to Catalonia, but that's a topic for another day) that either of these will gain independence in the modern day.
Realistically, it could be possible post-Napoleonic Wars (unlikely because the GP's probably didn't want a partitioned France), a post-Franco-Prussian War (unlikely because it would draw Great Britain's ire), post-World World I (ala Kaiserreich, but that would require German military domination throughout the conflict), or post-World War II (the aforementioned plan to split off Burgundy and Brittany, though the former would be against the spirit of the OP as Hitler wanted to annex Burgundy, and probably had little interest in spinning Brittany off as an independent state.)
