whats a good lvl to start working on jboots?

yep what lvl ?

Max. Because clicky run speed does not return more power than a level would, and sow pots exist from the start.

when you can kill the AC and farm the rest of the stuff afterwards (no rent rapier)

pretty much this ^ - he's hard to solo even at max lvl, so def have a group for it as there will be plenty of groups trying to snag it from you. Unfortunately there are douchecanoes that will kill steal as well

When you have 1krono and can swap for the ring.

Any level before 61, at level 61 with GoD unlocked you can have runspeed 5 which is 35%, the same as JBoots.

Grouping early for AC can be nice as long as your group is efficient. Gonna probably put a dent in leveling time, but that's your choice. The money isn't half bad.