The intended rate of levelling up in D&D 5E (as explicitly stated in the rules) is:In D&D, its kinda slow to level up. IMX, its slower leveling up in milestone variants because usually even a whole adventure may not satisfy the DM's milestone requirements. But even with the official xp variants, it still takes roughly 1-2 full adventuring days to get a single level up.
With that in mind, its very difficult to maintain a campaign that spans 5-7 adventures since creating a single adventure takes so much effort even if its pre-made.
Levels 1 & 2: gain a level after one session (4-hours)
Levels 3 through 20: gain a level after two to three sessions
So, to gain level 20 should take 36-53 four-hour sessions. You could do it in a year.