How far away before a turkey won't spook?

[JohnD.]

I'm just curious about this but how far away do you think a turkey has to be before he won't spook? Reason I ask is I frequently glass for turkeys in a huge crop field and I believe if they are in the 600-800 yard range they don't show a lot of concern but inside of around 500 yards they definitely get nervous. Just wondered what everyone else's experince is. Thanks, John.