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.