When I was on a ketogenic diet, a protein sparing modified fast that is (Rapid Fat Loss diet), I would have a half-day carb-up in which I would consume a shit ton of carbs in a five hour window with little fat and protein, 300 to 400 grams of carbs, and I weighed 230# when starting (quite fat).
It depends on how restrictive the diet is during the week and how lean you are. Go with an educated guess at first. Try 200 at first. If you continue to lose with that, increase it by another 25 to 50.