Well i've been DMing a campaign for close to 6 years non-stop now.
Ingame, 20 years have passed since the first characters have been made.
I have a paladin PC that started at age 25, now he's 42 and getting old.
I built the world myself, with all the politics and stuff.
The players have changed the world a lot, too. A few of them have strongholds, alliances, important roles in the society.
The world itself has evolved a lot. It started a post-apocalyptic demon-devil invasion. Human society was rebuilt from scratch, with help from the PC's.
New religions have been founded, magic has been restored, new cities have been found and united under a single banner.
Devils have invaded the society and kept control for years. Characters have helped liberate the capital city with a long and hard effort.
Not all the original players are present. Some have left, and have been replaced.
The nice quirk is that this campaign world has been ENTIRELY sparked by the same players, about 8 years ago, playing another campaign.
They were carrying powerful artifacts while exploring the tomb of horrors, trying to seal a portal. Of course they all died, and the artifacts had been lost. A demon found them, and used them to open the portal permanently.
Demons and devil invaded the world and decimated humanity. 10.000 years after, this new campaign started
I don't think i'd like to play any other world at the moment, as this is the world i've put years into. The players like it.
Sometimes we do sub-campaigns.
For example, they have played for 8 sessions as an evil party in the same world-year. Last session, they got killed by the very same "main" PCs, thereby resuming the original campaign.
We will probably play a few sessions in another age soon; the players have asked to see the opening of the first portal themselves, probably as demons.
Soon, they'll be able to use a flying ship planar engine to reach Sigil, and another sub-campaign will start in Planescape.
I didn't plot any of this from the beginning, except very general things like "main city will get invaded" or "demons will attack this place someday" or "this new technology will be discovered soon". My players shape the story, sessions by sessions. Like the first characters created the campaign, they are modifiying it each day. I think they will soon start a revolution, and i will not stop them from changing the world's order.
Yes, to me DMing is my own reward. But only if i am a narrator telling a long, convoluted story.
Oh and, if anyone is intested, you can google translate the world's story at
http://dnd.grieco.org.
It is in italian, unfortunately.