Exactly, but that's what makes counting hours or sessions pointless. There is no "typical" for my groups. Never has been. Same groups that's slow now may barge through the next adventure in 2 sessions instead of 4.
And before you say it all evens out - yes, most likely it does. The time in which they gain levels can certainly be averaged. But the poll doesn't ask for average. Nor does it take into account that while in an individual group things most likely average out, every group is different.
I am currently running 11 groups (yes, as I am stuck in the house most of the time I run a lot of online games and a table group). If I go by the poll, I could click all the options and then some because they are all at a different pace.
I generally run 3-4 groups (2-3 table & 1 online) in various systems and they do have different levelling rates, but 5 sessions/level seemed most typical.
If you read my comments below the poll you'll see I was asking for an average over the campaign, not a mandated amount for each individual level.
I think it's ok not to have a discernible typical levelling rate; back in the day my 1e games didn't - individual xp, I recall PCs who would solo a big monster and go up 2 levels (I hadn't noticed the 'only 1 level' rule...)