It depends on various factors. If your birds are already regular visitors, then usually it takes 2 -4 weeks before birds are getting used to your new Birdmill. It also depends on the position of the Birdmill; If you hang it too near to your house, then it could frighten birds off.
Make your garden as 'bird attractive' as possible. Put out bird food regularly; maybe have a bird bath to attract birds to drink and bath; the greener (plants/trees/bushes) your garden, the more birds you will attract.