Use the output of the Clock as the control signal for the switch. Then the threshold can be set to the time you would like it to switch. Specify Greater than the threshold or greater than or equal to to get the desired behavior.
If the switch is inside a subsystem and the clock is at the root of your model, you may need to add another inport to your subsystem to pass the signal into the subsystem.
Alternatively, the clock can be added to the subsystem and directly connected to the switch. The clock does not have to reside at the root of the model.
