Since you are using DIY, there is no real way to shut if off. Well, I guess that you could unplug the hose from the canister, but that would get to be a pain....
Anyways, you will need to keep track of your PH for the first few weeks, until you know how long the mix will last for. If you allready have a low KH, you may want to buffer the water with some crushed coral, or baking soda, as to prevent a PH crash. If you have a high KH(like me), then you won't have to add anything to the water to prevent a crash.
As the co2 levels increase, your PH will decrease. When the solution starts to produce less CO2, your PH will begin to climb. The problem with DIY IMO is that is becomes unreliable, and you have a PH swing every few weeks. I use 2 2L bottles to try to prevent this. I run both at the same time, and switch them out alternately(bottle 1 gets changed first week, bottle 2 gets changed the next). This will prevent most swings, but is not totally reliable. You probably should check your PH every few days, until you get a feel for when the CO2 starts to run out.