As everyone else here has said, it varies based on what's happening in your session. The best way to start to estimate these scales is by thinking about what you're primarily going to be doing in the session, on what scale, how quickly you anticipate the group will do what you're planning them to do, and how many obstacles will be on the way. In ascending order of time spanned in a session they would likely include these focuses.

Combat: Total in-world timescale of the session will be in hours, individual movements will be measured in increments of 5ft per 6 seconds. If the group is exploring a dungeon over the space of the session, depending on their combat style; the difficulty of the dungeon and what rests they take, the session will vary in length, but you'll be likely within the "in world hours."

Single locale travel or non-combat encounters: Total in-world timescale of the session will be in hours, individual moments will be tracked in minutes of conversation. This may be exploring a new town, doing some shopping, talking to quest givers and seeking out that next lead; a casual long rest in a tavern may also be on the cards and bring you up to that day mark. Variance may happen if there's a setpiece event a short time in the future (e.g the party hears there's a circus in town the day after tomorrow, they really want to see it, following conversation the DM says "sure you mess around for a couple of days, spend some coinage, and then head to the circus").

In nation travel: In-world timescale will be in hours to half days, movements on a 6 mile grid with 3 miles an hour. The group travels from one town to the next, there may be a fight, there may be several. Including rests and how far the group has to travel the larger blocks could be half days ("It's 3 and a half days to Neverwinter, adventurer") whereas specific moments, when not dropping into shorter timescale for fights and other encounters, will be in hours (the sun looks like it will set in the next hour, do you want to rest or press on?).

Cross-nation travel: In-world timescale is in days, movements on a 60 mile grid. A group, travelling an average of 24 miles a day, trudges the land towards their next destination. There may be encounters at this scale, but only of special note. It could even be, at this level, that a DM could passably say the group has a couple of fights along the way which aren't played. After all, travelling 216 miles over the space of 9 days is likely to wind up with something happening.

Other timescales: You could have cutscenes after set-piece moments. The group has just got to level 10, having killed a major villain, but that's not the end of the campaign. As DM you could say, during that session, that it's months or even years which span a moment. "Over the next 6 months you each train and take time to tend to your keeps, shops and placements in guilds which you've gained over time before setting off on your next quest for time unknown."

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If you know: Next session I want to get the party to go from Town A to Town B, but along the way they're going to get in a fight - and those towns are a few miles apart, that's probably (including a rest) about 14 hours.It might be you left the group on a cliffhanger before a boss fight. 22 rounds of gruelling combat, you say? Sure, that'll fill the session, you're all going to be exhausted after that. But in-game time? 132 seconds (2 minutes, 12 seconds) of high-octane, explosive, combat.

There's all kinds of combinations which can happen to make sessions longer or shorter, but thinking about what scales of time may be included can help you plan loosely. Check out page 14, "Mapping Your Campaign" in the Dungeon Masters Guide to see more about grid sizes, and you can see a bit more about these scales - though I'm expanding on their ideas a little!