In terms of deadmines, you can run about half way to smite and keep killing all and still get xp. Hope that gives you an idea. Or another example, in SM library, you can park alts at the door to the quadrangle (where Locksley is in the room off), and kill all the mobs in the square and your alts will still get XP. and another, in Cathedral, if your alts are parked at the door in front of the fountain, the limit is around by the steps up to the 2nd level.

I'm not good at estimating real world distances, but its around 80 meters/yards?

EDIT: As a side note, In case you don't know, levelling with only 1 alt and a 60+ is slower than levelling with multiple lower levels, e.g 1 70 and 4 10s in Deadmines. It works on average levels for xp in instances, so inviting along guildies or even randoms for the boost will speed the XP gain.