Campaign length
The average "campaign," meaning a series of linked adventures including many of the same Player Characters and a defined face-to-face group of role-players, typically lasts about three years for me.
Beyond that, people move around, graduate/get married/have children, or drop out of the gaming group for other reasons, including personality conflicts, exhaustion, spin-off groups, or (very rarely) their P.C.s reaching demigodhood. I'm right now between campaigns in the Boston area, and I've been recruiting new gamers to round out my group...
On the other hand, if you define a campaign as a setting where numerous P.C. parties pass through but that has long-term plot threads, Non-Player Characters, and location-specific encounters, my worlds are all linked and are independent of rules system, although I've preferred D20 lately. For example, here's a timeline:
-c. 1982 to 650 B.C.E.: "Vanished Lands" (heroic fantasy)-typically run with "Dungeons and Dragons" (started with AD&D1, now on D&D3.5, borrowing heavily from published worlds)
-c. 1787 to 1930 A.D./C.E.: "Gaslight Grimoire" (steampunk/fantasy)-typically run with GURPS (now in Fourth Edition), "Castle Falkenstein"; could also use D20
-c. 1930 to 2050: "U.N. Task Force/the S.J.I." (espionage/comic book superheroes)-run with several systems, including Top Secret SI, DC and Marvel RPGs, and GURPS; considering using D20 Modern, Spycraft, Silver Age Sentinels, or Mutants & Masterminds
-c. 2050 to 2150: Cyberpunk era-typically run with "Shadowrun" (Fourth Edition coming), but GURPS, D20 "Big Eyes, Small Mouth," and "Jovian Chronicles" also suitable
-c. 2150 to 2500: "Vortex" (space opera)-run with eponymous homebrew rules and GURPS, but considering various D20 science fiction supplements
-c. 2500+/-: "Immortals/Voyagers II" (time/dimension travel)-co-ran this, mostly with GURPS, but "campaign" is setting-neutral, enabling cross-genre and G.M. collaboration, as well as high-level adventures across the Multiverse...
If one takes all of these as a continuum, I've had close to 400 Player Characters use various rules systems explore my "campaigns!"