Is it possible to calculate the time required to empty or fill a pipe network in WaterGEMS and WaterCAD?
One of the underlying assumptions for the EPANET solver that WaterGEMS and WaterCAD is based on is that the pipes are always pressurized. Because of that, you will not be able to directly model a pipe emptying or becoming partially full.
If you're interested to know how long it takes for a tank to fill or drain, set up an extended period simulation (EPS) and let the tank drain or fill. You can then observe how long it takes for the tank to drain or fill.
A better way to model this type of situation is to use SewerGEMS. SewerGEMS allows for the modeling of partially-filled pipes that are running under gravity as well as pipes that are pressurized. In SewerGEMS, you would be better able to monitor the timing of when a pipe goes from the being under pressure to partially full, as well as how long it takes before the pipe is empty.
If you have PondPack, you could model this using the Pipe Volume pond type (or define the pond as an elevation-volume curve if you can determine the storage volume of the pipes in question at each elevation), with pond outlet structure set to the orifice(s) that release water. You can then use the Minimum Drain Time tool to see how long it takes to empty the
You may also want to consider the transient impact of expelling all the air from the system: Modeling the filling of an empty pipe in HAMMER
Modeling Transmission line priming starting from Empty conditions?
Time Required to Empty the System