You'd have to be a lot more specific about age and place. The minimum safe distance increases depending on the range of their weapons and how fast a sallying force can be expected to move.

But if you just want a random example where we have an idea of the number, then Julius Caesar's circumvallation at the siege of Alesia would have to have been located 2-3 km from the town he was laying siege to. He doesn't give the exact distance, but does give the circumference in his Commentaries on the Gallic Wars. The encampments (his army was split into several camps) would have been outside that defensive line, not too far away as he also had an outer wall built to protect against outside interference.