Depends on the group. I started running a campaign pretty much when 4e came out. The group is now level 11, close to 12. We play about twice a month (probably more like 20-22 sessions a year due to cancellations and such). Sometimes it can be 6 sessions without a level. This group is quite casual, and plays very slow. At low levels (1-3), I was able to squeeze in 2-3 combat encounters a session. Nowadays, I will rarely have more than one combat encounter a session, occasionally two, sometimes zero. I don't expect this campaign to ever reach level 30. I will continue to run as long as there is interest, which should get them to level 17-18 or so when 5e is released. Our 3.5 campaign that we started when 3.5 came out, hit level 14 when 4e came out. Seems about par for the course.
My next campaign (5e?), I intend to run only the even levels. Everyone starts at 2, and gains 2 levels at a time. I think I'd be able to get players to level 30 that way, before 6e comes out.
For a group that plays say 30 sessions a year, and levels every 3 sessions, a 1-30 campaign would be 3 years, which sounds pretty reasonable. In the 5-6 year life span of an edition, that could be two full campaigns.