I tend to track my #sessions and run longer campaigns, a couple years is typical.
Some recent campaigns:
Loudwater (4e) 2011-2016, 5.5 years, 103 sessions, levels 1-29 & reached 30 at end.
Follow on 5e Princes of the Apocalypse campaign runs monthly and plan to go 36 sessions over 3 years, 2019-2021, with fast advancement level 1-20.
Primeval Thule I have run 27 sessions so far since January 2019, plan to run around 60 sessions over 12 months or so, level 1-20 at 2-3 sessions/level.
Varisia - Runelords of the Shattered Star 5e D&D, so far 83 sessions since November 2015, but took a break in 2018 and switched from tabletop to online. Most of the PCs are now 20th level.
Followed on from Curse of the Crimson Throne PF, 34 sessions Jan 2014-Oct 2015.
Nentir Vale 4e campaign 26 sessions 2017-18, level 1-6.
Karameikos Campaign (Classic) 25 months to January 2017, ca level 1-18 at about 4 sessions/level, ca 72 total.
Southlands Campaign (4e) 20 sessions 2011-12, level 3-10.
Yggsburgh Campaign (1e AD&D) was 45 online sessions before tailing off.
Wilderlands (online5e ) has run a few hundred sessions since 2015.
1e Rise of the Runelords was 16 sessions online before TPK.
I used to mostly run 20-35 session campaigns in 3e, but with 5e I find it fits longer campaigns going the full level range.
Adding up #sessions for the completed campaigns above I get 103+34+26+20+45+16=244/6=22 sessions average campaign length, but this is biased to the low end by not counting the long duration ongoing campaigns like Runelords (83) and Wilderlands (200+).