I know that was the claim for 3e, but, honestly, I don't think that it was all that achievable. I think it was a lot closer to about 2 years to go 1-20 (and possibly 3 years) in 3e.
I don't recall the PCs ever levelling slower than once per 3 5-hour sessions in my 3e campaigns, even when they were up around 17th level, and usually faster than that. There was a period ca 10th-14th when the frequent death & consequent loss of levels stalled advancement, though. For one 3e campaign I went over to 1/2 XP, which gave a 1/5 sessions rate - that was for Lost City of Barakus, which is designed that way.