Sounds like a really interesting brew.
That uses Wyeast's Roeselare blend. I haven't used that blend before but it has similar components to their lambic blend so I think some of the conventional Lambic wisdom should apply.
I'm about two months into a Lambic roughly following the Steve Piatz method in BYO:
He leaves them set in plastic buckets on the yeast cake for two years. This is similar to what commercial lambic brewers do. They use oak barrels. Oak and plastic will let a little O2 in which might contribute to some of the sour compounds.
I'd leave yours in the primary for as long as possible tasting about once a month to see how it is doing. If you are running short on time then I'd go into the secondary about a month before your deadline. I think the oaking will go faster than souring.