This is up to you, but the best time for most men to use a face mask is in the evening before bed.
This is because you need a time window in which you will not be disturbed to set up the mask and do its work.
If You use a face mask in the evening before bed, your face will have a number of hours of constant peace before returning to work and the outside world, where the weather is exposed to dirt and debris.
You can use masks in the morning when it is needed, but this strikes the target, as your pores will soon be hidden again.