I'll preface this by saying that I give a significant amount of xp to my players for role playing and exploration, as it makes zero sense to me that 100% of the xp should come from only one of the three pillars of the game, especially when it's not even the pillar that sets tabletop role playing games apart from most other game types.
I'm currently running two games.
My oldest games is a year old and has 3 PC's, although they had a flesh golem from about levels 5-7 and I divided their xp earnings by 4 instead of 3 during that time. We play once a week for about 4-5 hours on average, and have played about 80% of weeks during that year. We only have 1-2 random encounters per session.
The PC's are now level 12, and aside from going from 11-12, which I think took less than two full sessions, the leveling has felt pretty steady overall, with the party leveling up every 3-4 sessions. I think part of the reason for this is that the party can handle much higher CR amounts relative to their levels once they got into the +3 proficiency tier, and as a result were able to maintain their leveling pace even though the xp requirements went up dramatically. I remember one fight they had at level 10 was against a mummy lord (CR15) and about a dozen mummies, resulting in a CR roughly 3 times their level. This is a feat they just would not have been able to replicate at level 2.
My other campaign has been going 4 months, and we've only missed 2-3 sessions with 3 PC's. They are all either level 6 or nearly level 6, which is putting them at a level up about every 3 sessions (again, which plenty of extra role playing xp, as they are big on the role play).
I think one of the biggest effects on level progression is how the encounters in a given campaign are constructed. That half-dragon veteran might be roughly as tough a random encounter for your 5th level party as 20 goblins, but he is going to yield them far more xp.
To actually answer your question (just realized that most of the above stuff isn't really what you're looking for), my 12th level PC's feel *very* strong, and I throw really high level stuff at them to keep it challenging, so I imagine they are going to move very quickly toward level 20. Our last session had them easily crushing two CR10 critters with very few party resources used, resulting in nearly 4000xp for each of them, which as level 11 characters at the time meant that one easy random encounter got them over 1/4 of the way to 12.
