I'd start with .5-1 ouces for five gallons and go from there, leaving the cubes in for a long time(couple months at least). The oak character and complexity is better I think, there's a reason wines and beers are aged on oak for a while and not just always dumped into 100% new oak for a couple weeks and removed. Also, you don't want the oak overpowering the saison complexity/character, so I'd go easy.
"Did you ever kick a woman in the midsection while she was wearing a whalebone corset? I nearly broke my great toe. I never had such a painful experience." -Egbert Souse(W.C.Fields)