To Use Oak Cubes or Not and When?

So I've got a fun little experiment going in my basement. I've had a sourdough starter that I cultured from the air and have had going for ~ 6 yrs and then subcultured for a week or so in sterile wort and got a good starter going. I made up a simple Saison and divided into 2 1 Gal. carboys. 1 got sourdough starter and another got sourdough starter + dregs from a Goose Island Matilda to ensure some Brett. They've just been chugging away and I plan on not touching them for a year (now around 8 months), but was thinking about chucking in some oak cubes and pulling off a gravity sample then while I've got the carboys open. For just a touch of oak flavor how long do you guys recommend leaving the cubes in? 1 month? 2?

Thanks!