My first on-going comic, True Power, was about 500 pages total and ran from March 22, 2007 to Sept 24, 2016. So about 9.5 years, but I had a 2 year hiatus during it so it took about 7.5 years.
The sequel, True Power: MS2, started May 14 2017. I've completed 76 pages out of 380, so there are 304 more to go. I would like to complete it by next year.... we'll see... I'm hoping to at least get up to 3 pages a week.
My main comic, Celestial Chronicle Shion, started the same day as TP:MS2, but it will be going on for much longer as it has more pages and the pages are more detailed so they take longer to work on. I can finish only one page per week. I expect it to go on for at least 6 years. I haven't finished the thumbnails yet so I don't know exactly how many pages it has and I keep losing count as I redo them and adding in or sometimes combining pages!