When a monster drops a health globe, how long will it last if not used? Should I leave unused health globes behind as backup in case I come back to the same room / dungeon floor later?
My findings have been interesting. It seems to be affected by how far it is from you and whether you are indoors.
If you are within about 1 and 1/2 to 2 screens' distance away from it when you are outdoors, then it dissipates in 60 seconds. This distance factor seems to be a lot smaller when you are indoors.
If you give sufficient distance, then it seems to last for a long time, as long as 50 minutes in one instance before disappearing.
Globes do not disappear when you exit the dungeon that you are currently in (either by portaling or exiting the current dungeon) and return later. They do disappear if you quit your current session, however.
I am going to stop testing as I doubt anyone would wait for more than 50 minutes to go back to a mere health globe. Boss fights last longer, but since most boss fights are fought in a constrained area, it's best to use the health orbs within a minute of them appearing as you are unlikely to get beyond two screens from them.
I counted in a few videos—they last around a minute before disappearing. It wouldn't make sense to leave them behind as reserves.