Replying to Sword of Spirit:
I like these, but I generally prefer to go for 3 years not 4+. I finished a 5.5 year 4e 1-29 campaign last year (Loudwater) and in retrospect it was too long, should have gone for 1-20 over about 3.5 years.
I also have run several ca 2 year games of around 30-35 sessions (eg 3e Lost City of Barakus level 1-8, 3e Willow Vale level 1-8, PF Curse of the Crimson Throne level 1-14), which I think are large mini-campaigns in your metric. I would tend to regard these more as full campaigns, albeit on the short side, but not failures. I ran a 12 session level 1-4 Pathfinder Beginner Box Yggsburgh game, which was designed and completed as such. I would call that a true mini-campaign. I have also run a couple 4e campaigns (levels 1-8 and 3-10) that each lasted around 18-20 sessions and could be considered failed campaigns, having been conceived to potentially run longer.
My current campaigns are:
1. 5e Wilderlands - long running open world sandbox game since at least 2009, longest lasting current PC Hakeem the Barbarian-18 started Jan 2015 when we rebooted in 5e rules. No end point, but current major story arc will likely be another year or so.
2. 5e Varisia/Runelords of the Shattered Star - started November 2015, 50 sessions in, expect to run about 70-90 more sessions over about the next 1.5-2 years, so aiming for about 3.5 years total.
3. Just announced my Classic D&D Karameikos game has ended as a group tabletop game at least for now. In current form went from start of 2015, so 2.5 years, going from level 1 to about 19th.
4. 4e Nentir Vale, 6 sessions in, intended to run around 50 sessions fortnightly over about 2.5 years, go 1st to maybe 14th level.
So it sounds as if I prefer slightly shorter campaigns than you, but similar principle.
Speed of levelling in 5e is definitely a potential issue. My solutions are (1) to run at 20th with Epic Boons, plus Feat and ASI gains every 50K XP and (2) Individual PC XP, which has a surprisingly significant effect in slowing down group advancement. One high level PC in a mid level group basically has to play a mid-level game, totally unlike 5 or 6 high level PCs.
Edit: Definitely disagree that 4e XP levels PCs too fast. Default 4e advancement rate is glacial, especially from 11th level up. If you use the DMG2 systems, tons of minions, and generous quest awards, you can get it up
to about 1 level per 12 hours, but by default is nearer 1 level per 20 hours' play. With default XP my 4e level 1-29 campaign would have taken 8 or 9 years rather than 5.5.