as soon as you get wind blast rank 6
if you get a good DSK sword out of synthesis or blacksmithing it can have 106 max (+59)
Brionac's max damage with spark is 239, which is the same as a maxed ego dustin sword
I seriously don't see any point of this sword beside skill training and mainstream requirements.
Wind Blast training is light on the AP, it just takes several several hours to train up.
Synthed DSKS are much harder to enchant on. Either using an auctioned DSKS (Rich Croc) or a gacha one (D.Lich Spike) then following it up save a lot of trouble. Otherwise I actually would just go with the Jousting Claymore, because at least that one is free.
Even with a whopping 239 max (assuming you have Spark ranked that high) I still wouldn't use it. AP is precious, I don't think the extra damage is that significant to warrant wasting actual character development.