use gas law properties to determine how long the keg will keep pressure. if you have a 5 gallon keg with 4 gallons of liquid in it, and you pressurize the 1 gallon of headspace to 10psi, then you would have 5psi in the headspace after you draw off 1 gallon...
1gallon of headspace @ 10psi
=2gallons of space @ 5psi
=4 gallons of space @ 2.5psi
...etc
if your keg is filled all the way to the top, you might only have a pint of headspace. so replace "gallon" with "pint" in the above example. so you will have 2.5psi left after only 4 pints are drawn off.