Zardnaar
Legend
Yeah, levels 1 and 2 each only take half an adventuring day to gain enough XP to level up. By design, because they’re supposed to be tutorial levels. Should be about 3 combats each if they’re designed according to the encounter balance guidelines, and most levels after that last for about two adventuring days. Ish. Most levels take around 10 combats to gain enough EXP to level up, a few take 12, and if I recall correctly 5th level takes 15.
20 rounds of combat is really not that long. Again, 3 rounds should be doable in about 10 minutes, maybe 15 for a slow group, so 20 rounds should be doable in an hour or two, leaving a typical D&D session with an hour or two for exploring the dungeon between those combats and and hour or two of interacting with NPCs in town before and/or after.
Do you DM 5.5?