I have two homebrew campaigns. I started my first in 1981, essentially when I began playing D&D. I started the second around 1993 or so, a few years in to second ed. AD&D.
My original campaign started for two reasons: characters and consistency. I set each scenario after the prior one, and tied them in as I went along, whether I was using a published module or self-written. In turn, my group used the same characters, or assembled replacements in the same group and context. Within a year, I was looking for a map to start placing the locales and new story ideas prompted by earlier sessions, and customized a map of Europe (which was later revamped again when I converted the campaign to Runequest and then T&T, and years later back to AD&D 2nd edition).
In a large way, the depth of that campaign and its life sprang from the fact that the same key characters remained in the game, some of them becoming important NPCs after the players had to move on; likewise, by building a consistent setting for successive adventures, it allowed the skeletal framework to take place. Years later, in 1988, I moved on to college, and the game got a revival with a new group (1 player, my sister, from the old) and a facelift; I revised all of the old material, set the campaign 80 years later than the last series, and used that time period to tweak, modify, and use history to explain away/modify things which had grown weak or inconsistent. The end result was a blast of a campaign to run throughout my college years.
In 1993, still plugging through at the University of Arizona, I had become enmeshed deeply in my major of Anthropology, and I was getting a serious bug for trying my hand at a setting which was whole cloth, and which held cultures and story possibilities that I could design around my academic interests. My other world, of course, had been continually tweaked, but to remain faithful, it still had a certain lay of the land, geograpnhy, and set of cultural assertions that were hard to break without changing its own flavor. By starting a new setting, I could manipulate all of these fatures as I saw fit.
I started with a 30 or so page gazetteer, a map, and some concepts derived from an earlier mini-campaign I had experimented with. I didn't define everything; just the 5-6 locations I wanted to start the game in. I used real-world models for every setting initially, then went back and thought of a couple key elements that would give each a unique spin. I looked at conventional races in AD&D, threw out the conventional notions, and brought in unorthodox interpretations, while introducing previously unattainable races as PC types. The end result was a carefully tailored mix, which I have continued running to this day. I have a couple players who have now stuck with that campaign long enough to make it to 3rd edition, with 24th level PCs. That campaign has grown from a 30 page gazetteer and one map to a 200 page GM's guide and 50 page player's guide and approximately 7-8 maps (poster size). I've run about 5 campaigns in this setting, the longest running 9 years, the average running 2 years.
Of course, my original home campaign fills 5 spiral-bound volumes, 3 200+ page GM's guides, and over 40 poster maps, and a total of 19 campaigns, the longest 10 years, and an average of 5 gaming hours a week for 20+ years.......but honestly, I ran that game when I had far more time, usually, and the campaigns could eb very far reaching in scope. The newer world setting is more tweaked, and games often run in a single region fo a year or more, and sometimes never leave that area.