How frequently do you play and how long are your sessions? Do you use XP or milestone leveling? Just curious.
Right now I am playing 5 regular games (4 as a player, 1 as a DM)
3 of them are 4 hour sessions once a week (including the one I DM). One of them is a 3 hour session once every two weeks.
The one that is 3 hours would take longer than 6 months for a 1-20, but it would not take years.
Usually we are playing milestone now, although we did play XP in the past.
My campaigns have 8 hours of playtime in a month. I don't see getting through 20 levels of 5e play in 6 months with only 8 hours a month. Maybe if it were every week. 4-hours a week is pretty common, but my impression is that is usually enough to complete one of the WotC adventure books in 6 months, ending at level 10 to 15.
The game I just played tonight as an example, started on March 20th and we hit 13th level tonight. We also had a TPK in there at level 3 (and restarted at level 3 with new PCs the same session).