Mobs will chase you from a reive area until you lose status or max enmity on them. I'm not sure how far they go, as they deaggro once you lose Reive status.

there are some situations where it would be optimal to escape the current reive. I saw a terrible wipe of JP players the other night. I was on THF and amongst the last of the living and all the mobs came for me. I didn't make it out of the reive in time. Had to eat a HP and lose travel time, etc.