Patient leaflet:

There is a leaflet found in this link, that does not mention a duration on how long to hold the breath. There is also a video that refers to an apnea of a few seconds.

A review [1] published in the European Respiratory Society Open Research journal states that in order to enhance drug deposition in the respiratory tract, apnea should last ideally 10 seconds, and a minimum of 4 seconds. If not possible, it should be considered the use of a spacer device.