Because I have a player pool and more than one character per player that can be swapped in and out, it's a little hard to say. Based on the most consistent player and character in my town-to-dungeon campaign (two campaigns ago), it's probably something like 80 hours of play to get from 1st to 8th level. That campaign was exactly 20 sessions, each 4 hours long, over the course of 6 months. In game time, this was 20 weeks.
In my current Planescape campaign, it was consistently 8 hours of play to get from 1st to 3rd level which is consistent with the aforementioned town-to-dungeon campaign. From 3rd level to 7th level, the most consistent player and character would probably have clocked in 14 more sessions at 4 hours each or a total of 56 hours of play. That seems about on track with the pace of the other campaign.
I also play in a Tomb of Annihilation game where we started at 3rd level. We're now approaching 8th level and Session 19 is this Friday. Assuming we rack up some XP this session and level, we're looking at about 15 hours per level. Which makes some sense since we skipped 1st and 2nd level. The speed at which those levels are attained would skew the hours per level lower.
So yeah, probably clocking in at 10 to 12 hours per level on average from 1st to 10th level, more or less, using standard XP advancement. Or leveling on average once every 2.5 to 3 sessions. There are five PCs per session, usually. I imagine the average time per level will continue to drop past the "sweet spot" as I understand it's faster leveling as you get into the higher tiers.