2-3 weekly sessions is about a month. Also critical to that snooze is that 5e simplified combat to charge in & hit it with your hurt stick with little to no mechanics supporting any meaningful strategy allowed by the missing tactical component once handled from move based AoO's, facing, flanking that comes at a cost, & so on.
Like AD&D - but if fits within the basic framework of the design of the game. The important decisions to be made are strategic, not tactical. If you're trying to use it for exciting tactical combat then you're using the wrong tool for the job.
It can also jump to 3-4+ if they spend a bit of time exploring & such. It's one thing to make the occasional interesting dungeon jam packed with baddies as you suggest for a big thing, but doing it month after month is a huge task... I don't evem think many of wotc's own dungeons are packed with 12-14 encounters.
Shrug I'm not saying it's the best way to play an rpg. I'm just saying that this is what 5E is designed to do. (Although it doesn't have to be a dungeon - it works just as well with a ruined city, haunted forest etc).
Don't forget that you need to have some way of making sure the PCs can't just move some bodie & rest there.
Same as it ever was. 5E is slightly better in this regard. It at least signals that a long rest is a somewhat arbritrarily defined unit of time that can be adjusted.
But really if 5E is not being played in an environment based way then it really suffers in comparison.
If I want to run a game where I have 3 combats I want the party to face, and they're increasingly elaborate set pieces with a big final boss fight that I have some confidence the PCs will be able to handle (but won't curbstop), then the tools 13th Age gives me are orders of magnitude better then anything that 5E provides. It's not even remotely close. The encounter design is simpler, faster and vastly more accurate, the fights that result are tighter and far less swingy.
Having run both games back to back recently it seems clear to me that if I'm trying to run 5e like 13th Age I'm just being dumb. Where 13th Age struggled is with the optional encounters when I tried to include a dungeon crawl - like 4E a relatively easy fight that just drains resources just takes too long (although it's much faster than 4e in general) and fails to really drain sufficient resources that are not recoverable - whereas such a fight in 5e is over very quickly and does potentially drain resources that will be missed - if you're actually using the guidelines.
I'm somewhat sympathetic to the argument that perhaps 5E chose the wrong goals for it's design - that they went overboard in their enthusiasm in going back to the roots of the game and taking on some OSR elements and didn't really understand where they went wrong with 4e, or what the majority of people actually want to do with modern D&D, but at the end of the day I don't believe in blaming a hammer for being a bad screwdriver.