Mine fly all the time but just across the yard! I have yet to have any leave me. I was worried about this when I first got them, so I got Rouens instead but have since been given some mallards. They won't travel away from their food!
I just got them last week, so they may not be very "attached" to me....that's what I'm worried about. For now they are very content with the water in their run (2 foot chicken wire)