You can, in essence, "flush" forever. You are supposed to flush until you get new fluid from each corner (and, ideally, from the clutch as well) Its up to YOU to stop then, though. You COULD go through an entire extra bottle for no reason.
Seriously, though: I would expect to use one or one and a quarter bottles (what's that? 8 oz, or 10?)
Just keep refilling the resevoir BEFORE you suck air! lol and pour SLOW so as not to introduce air while pouring.