After 11th level the amount of XP you need to level decreases.
You need 21,000 xp to hit 11th level.
To hit 12th you only need 15,000. To hit 13th you only need 20,000. To hit 14th level, another 20,000.
It's only when advancing to 15th level does your XP needed hit 25,000, increasing by 5,000 per level there after in a linear fashion.
Importantly over those levels 12-14, your XP 'yield'
increases (as you're fighting higher CR creatures, with an ever increasing XP budget, in addition to needing
less XP to advance), so thoe levels 12-14 go past surprisingly quickly.
When you hit 15 level and your XP needed finally rises to 25,000, your average XP yield from a Hard encounter is nearly twice as much as it was at 11th level (so you advance from 14th to 15th level nearly twice as fast as you did to get from 10th to 11th level). This pattern largely continues till you hit 20th.
In practical experience (having DM'd a campaign to 20th plus, using roughly 4-6 or so 'Hard' encounters per adventuring day as a median, and with 6 hour average session lengths) it's roughly:
- 1 session per level for levels 2-3,
- 3 sessions each per level from 4-10,
- There is a big pause to hit 11th level which takes roughly 4 sessions
- levels 12-20 take roughly 2 sessions each to hit.
Roughly 40-50 longish sessions to go from 1-20, or around 1 year of weekly game play.
A longish (6 hours) session with a reasonable amount of combat at 1st level can get you all the way to level 3 in a single session.