5e Campaign Structure
Dwiz points out that the function of long rests in 5e is not at all to simulate the effects of sleep than it is to pace the game, by way of the consumption and restoration of resources (esp.: hit points and spell slots). He finds that the "gritty realism" variant is better at this than the typical 8-hour rest rule, since it's more likely that the party will deal with approximately 7 encounters between long rests.
Gumbo looks at the encounter experience chart for general challenge experience. Not only can you use the guidelines for encounter difficulty to gauge the experience earned from completing a challenge, but you can use that same method to determine experience in hindsight of an encounter. You could combine this understanding with a simplified pip system for tracking experience, understanding that a deadly encounter yields from 1/3 to 1/4 the necessary experience to advance, and that less difficult encounters are worth from 1/4 to 3/4 the experience of a deadly encounter.
Then something clicked for me. The official guidance is that there should be 6-8 mid encounters per "adventuring day", which is really to say there should be 6-8 encounters between long rests (regardless of whether the duration of a long rest is 8 hours or 7 days). You know what's crazy? If a mid encounter yields 1/2 the experience of a difficult encounter, and a difficult encounter yields from 1/3 to 1/4 of the experience necessary to advance, then it should take 6-8 mid encounters in order to advance. This means that the time between long rests should be the time it takes to advance a level.
We can model the basic campaign structure of 5e as follows, using a simplified experience scheme:
1. Begin adventure with 0 XP. 2. Repeat until XP meets or exceeds 12 (or 16): 3. | Complete challenge. 4. | Gain XP from 1 to 4. 5. | Take a short rest if desired. 6. Progress to the next experience level. 7. Start next adventure after a long rest.
And that's all there is to it! The actual time between encounters and adventures depends on the nature of the campaign, which the 5e DMG actually lays out pretty well: the default 'heroic' pace on the scale of days, the 'gritty' pace on the scale of weeks, and the 'epic' pace on the scale of hours. The extent to which all this is self-consistent blows my mind.
There's one potential issue: hit dice, for healing. You get 1 hit die per level and can spend as many as you want per short rest to restore some hit points. But 1 hit die spread across 7 encounters, even if some of them are actually non-combat 'challenges', feels too little. Apparently Baldur's Gate 3 decided to be like 4e, allowing characters to restore 50% their hit points per short rest and only allowing 2 short rests between long rests. I haven't played the game myself, but I expect its structure to look a lot like Dwiz's diagram in his post, with there being up to 2-3 encounters per actual day.
I'd be inclined to have characters heal ~50% their hit points per short rest without there being a limit between long rests. Tracking hit dice is kind of annoying to me, and feels excessive when the larger point is just that you can recover a little but not all the way. A formula I suggest in FIVEY is d6 plus your level, which accomplishes this alright (especially while being friendlier to lower level characters, for whom every hit point counts).
As to what pacing I prefer, I'm inclined towards a cross between the default and gritty methods: short rests are ambiguous and automatic after each encounter, and long rests take 1 complete day (in a safe haven). To have the best of both worlds, I would even say it takes 1 week (& 100 coins) of training per level to advance after earning the experience necessary. Then this would also serve as a money sink.