How long should a blower and dehumidifier run in order to dry out ceiling boards and drywall?
The only real answer can be "as long as it takes". There are so many variables that affect how easily the water will evaporate that you can't predict. Could be hours in some situations, could be months in others.

You need to figure out how to measure how much water is left. You might consider buying a moisture content meter; there are ones with pins that you push into the surface (meant for wood, but may work with plaster), and inductive ones that measure through the surface (but may be less accurate). Or, you could figure out how to get to the wet area and check it out first-hand.

If the wet area is confined (e.g. between ceiling and floor) you should do whatever you can to get air flow through the confined area. For example, some Googling turned up this equipment which professionals use to inject air into a wall cavity to dry out the cavity. You probably don't want to buy one of those, but whatever you can do that has the same end-result will help.