When you put CO2 pressure on the keg, the CO2 build up in the free space above the beer. Eventually, some of this CO2 will precipitate into the beer. However, at the same time, the beer is also emitting CO2 (think beer in a glass with the bubbles coming up). Eventually an equilibrium is reached and there is too much C02 in the headspace for the beer to release it's CO2; thus the CO2 stays in the beer creating carbonation. However, as you pour beer, the headspace in the keg becomes larger and larger. If you do not have CO2 attached to the keg, the equilibrium between the CO2 in the beer and the headspace is broken, allowing the beer to release it's CO2 into the headspace. Eventually, there will be enough headspace (and not enough CO2), where the beer is able to release all of it's CO2 into the headspace and thus causing flat beer.
So, very long story short, you have to keep it attached to the CO2, or at the very bare minimum, hit it with a CO2 blast every 5 or so pints. I would recommend keeping it attached.