Fortunately, there is room for two in the VAB. I think they may wait as long as possible[IE, right before they start the countdown], to make a rollback decision. I am not sure how much a launch criteria a down range storm is. That is more of an issue for booster recovery ships. I seem to recall them launching with rough weather in the recovery zone. If Franklin stalls off the coast, which is starting to look less likely, they may have to consider rollback. As of right now, if Franklin does head NE in the short term, they should be able to get a shot in for a launch on Tuesday. Anything after that would be questionable if Franklin loops back. Although, if they fill the tank and try for Tuesday and do not launch, they may be in trouble considering the 24-30 hours to empty the tank and then seven hours[I think] for the rollback.

My guess for now, they start the clock on Saturday. If Franklin hints at a turn back, they stop and scrub the countdown and prepare for rollback, holding off for as long as the safely can.