The description of the Hex spell states (PHB, p. 251):
If the target drops to 0 hit points before this spell ends, you can use a bonus action on a subsequent turn of yours to curse a new creature.
The spell has a minimum duration of "Concentration, up to 1 hour" and possibly much more contingent upon spell slot level (8 or even 24 hours), so is there a limitation on what comprises a "subsequent turn"?
The way it is worded, it appears the Warlock could simply maintain concentration and keep moving the Hex around from one battle to the next (even maintaining it during a short rest with enough duration), so long as the previous target is reduced to 0 hp.