In my time playing and running 5e D&D, I've noticed that giving a spell a duration of 1 minute seems to be a polite way of saying "This spell lasts for the entirety of this encounter, but won't last into the next one."
However, there are other durations of spells: mage armor lasts 8 hours, conjure elemental lasts 1 hour, and shield of faith lasts 10 minutes.
In down time, an estimation of how long these spells last is good enough. I can just give a guess at what the wizard is able to accomplish with help from an earth elemental for an hour. But in encounter heavy areas (like a dungeon), it's a different story.
Even the shortest of these lasts an extraordinarily long number of rounds (100!), and I don't track time between encounters with that much granularity. So how do I know when these spells should run out?
Is there any sort of heuristic (like 1 minute = 1 encounter) for spells of other durations? Something of the form:
- 1 minute = 1 encounter
- 10 minutes = 2 encounters
- 1 hour = until a short rest
- 8 hours = until a long rest
Would be good. Those numbers were my first takes at estimates, but some seem inconsistent depending on which way you look at it (e.g. 8 hours is the length of a normal work day, but an adventurer has 16 hours of waking time a day (with 2 short rests mixed in), so does mage armor last all day, or do you need 2 of them?), and others are just a random guess (i.e. 10 minutes = 2 encounters).
As a player, I'd be looking for some sort of rule in a book, or developer commentary I could point a DM to. But, since that might be hard, as a DM, I'll settle for any heuristic that has been tested or can be shown to be reasonable within the system.
One could show that a heuristic is "reasonable within the system" by showing how a spell (or spells) of some duration is balanced if it lasts for some number (1, 2, 1.5, etc.) of encounters on average in the recommended 6 to 8 encounters a day ("The Adventuring Day", DMG pg 84).
In this case, "until next short rest" would be somewhere between 2 and 3 encounters, and "until next long rest" would be the whole 6 to 8.
Best Answer
In my experience, I've used exactly the numbers you've proposed with no problems arising. Specifically, spells lasting for the number of encounters suggested have seemed balanced against spells that last 1 minute.
To address your doubts about those numbers:
8 hours is reasonable for the amount of time a party spends exploring a dungeon or traveling. The other 8 hours (those not spent as part of a long rest) are assumed to be setting up and tearing down camp, cooking, tending equipment, etc. (The rules say this somewhere, I'll try to add a reference later. I believe it's in the DMG in the section about traveling.)
10 minutes is a reasonable amount of time to cover 2 battles in close proximity. If you assume (for simplicity) that the first battle takes place in the first minute and the second takes place in the tenth, that leaves 8 minutes to gather fired arrows, check a few pockets, put on dropped backpacks, head back into the hallway and kick down another door. That seems pretty reasonable to me, and has worked well at my table.
Obviously, any numbers you decide to use are going to be general starting points with regular exceptions. Long bouts of exploration, NPC interaction, puzzles, and shopping will obviously affect the time between encounters or rests.