Lambics are the only style that are traditionally aged in primary. The breakdown of the cells helps to nourish the slow waves of different yeast and bacteria that grow over the long fermentation.
I'd rack to secondary, depending on what your gravity is like, you'll probably just need a few months. Saison yeast is pretty attenuative on its own, so there shouldn't be too much for the Brett to eat. As long as the gravity is low/stable over a month and you are happy with the flavor you can bottle.