This depends a lot on the size of your room and the level of dampness. For instance, if you’re looking to dry up your very damp basement that has seepage, it may take weeks for this process! This also depends on your dehumidifier’s capacity, so it’s impossible to provide an exact value.
Generally speaking, though, if you want to alter the humidity of your overall living space, it may take you anywhere between 2 to 24 hours to do this, depending on the relative humidity and the capacity of your dehumidifier.