it wont go stagnant if you don't seal it up. let it vent/keep topped off youll be fine Ive gone that long before in between using change water or not.
many newer setups/new water RO DI units and new storage bottles don't have the living adhering/scummy organisms and contaminants that an older production system has, in the lines for example, so that's why sealing works well up front then one day the act of sealing the change water kills an entire setup when o2 demand tips without telling us, and we input stagnant smelly water (did it, I vent from now on forever)