On one hand, the DM can control how fast a party levels up. If they want to run a campaign were leveling up occurs over the course of weeks, months, or years, they can.
On the other hand, all official material assumes the party is going to be leveling up fast, sometimes even multiple levels in a single day.
Which of these two leveling paces is the expectation for most players? The answer is fairly obvious. If WotC really wants to make downtime an important part of play, they should make adventures that lean into its usage.