I would say there is a little more than 1 gallon of headspace. I used a glass carboy. I think it is a 6.5 gallon. With 5 gallons + a 750 ml bottle of makers in it. Seems like I will probably leave it for a while then keg it and let it develop after giving it a shot of co2. Was hoping not to have one of my kegs tied up that long but oh well. Thanks.
Since you can purge the headspace, you should be fine leaving it in the carboy for that long. Just make sure you keep the airlock full and the temp stable, otherwise you’ll have oxygen getting sucked back into the carboy if temperature drops. Also, the rubber stoppers are less oxygen permeable than the silicon universal stoppers. And make sure you purge it again whenever you open it to sample or what not.