Once theres no more bubbles get rid of the mix, wash out the bottle and start over. Best thing to do is to have a 2nd 20oz bottle ready 2-3 days before.. so all you do is switch up and theres no drop of co2/pH.
once you open the cap off the bottle the pressure will drop (duhh lol) but since you already have a bottle thats been ready and producing co2, the turnaround will be much faster, minutes instead of 12-24hrs..Interesting....I take it by having it pre-mixed and ready to swap over, there will not be any issues of pressure?