We've had at least six campaigns in my home-brew Iron Crown campaign. They've spanned around 6 years. We started in the city of Hollowfaust in the Scarred Lands setting shortly after the HF book was published in 2002. The reason for the many campaigns instead of one single one was the uneven structure of D&D 3x. There are "sweet spots," such as around 5th level and another around 10th. Then the game becomes unmanageable and a lot less fun for me around 13th level. The best laid high level plans of DM's are often brought to ruin by simple arcane bypasses by players.
4th Edition vows to smooth out sweet spots, providing lots of fun at any level, including ascending to 30th level within the Player's Handbook. So for the Labash campaign, I'm stating my intentions now to run the first campaign to 30th level, or at least try. I was usually the one responsible for instigating a campaign re-boot, so I figure I'll be equally responsible for keeping it together. I've never had a player quit because their characters were getting too powerful. On the contrary, there were quite a few arguments to keep going.
The concept of going from first to epic levels allows the campaign setting to be singular. I can state now that I only want to run one campaign in this setting. It is our playground and you're welcome to tear it up, create empires, vanquish evil, travel the planes, whatever you like. My investment in time to create the campaign should be well rewarded if we hit 30th level. I figure it should take at least 4 years, perhaps longer.