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."
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.