I would think that a logical first step to starting the job system is to assign each class their future job, then start to work on it. Therefore, this info should be known to the devs relatively early in the process, almost from its inception.
Those job/class assignments are actually really important to players. Many of us are levelling entire classes with the future job assignment in mind. That is, they're not playing thaumaturge to play thaumaturge. They're playing thaumaturge to be a black mage or red mage. Needless to say, levelling of that class takes many hours.
When job assigments for each class are known, could we get informed of that ASAP so we have time this summer to prepare for that?
If for instance, someone who loves blm levelled conjurer...but at the last minute finds out conjurer actually unlocks white mage...that's instant rage factor there. Or if someone finds out that black mage, or thief, or summoner isn't an initial job at all, that's rage factor too. And if say marauder gets beastmaster...that is bedlam right there. Players need time to cope with the possibility that their job doesn't match why they levelled that class, or that their job isn't even in the first round at all.
Players will tolerate any potential disappointment/missed expectations the more time they're given to react to that news.
So earlier is better. Like...months in advance. As soon as possible. People need time to react to that decision.