Who has proof that the bees will use a perfect circle.
It is also known they "will" fly over 3 miles.
You will have to be in an area where there is no spraying the forest, where there is no power lines rights of way to be sprayed. What else do you have?
Lawn treatments, swimming pools, pest control sprays.
Bees go into some strange places to get what they need.
So call it 10 miles... and while the area the bees fly in may not be a perfect circle, the area the beek would have to have chemical free would have to be a circle out to whatever radius the bees have the ability to fly to. Either way, there are still vast areas of land in the world, and some of it may be chemical free, perhaps a huge organic farm, or many organic farms in one place... or maybe wilderness.