I think whenever you feel powerful enough. After you beat them a few times they will become laughably easy due to their low Mandate.
She's currently wiping the floor with the Chagatai-Timurid alliance. Not exactly encouraging.
Anyway, I still have to conquer all required provinces and I only have 1 vassal and no willing ally. Still choosing between finishing off Korchin and eating more of Oirat land (and vassalize her, if possible), but I'm worried about high AE and a coalition.