There's been a lot of wailing and gnashing of teeth concerning the upcoming 4th edition of D&D, much of it over how it's too soon for a new edition. I mean 3.5 came out just five years ago, right? Wizards of the Coast must be a greedy bunch of corporate sellout bastards! But are they really? What is the average time between editions for most role playing games? Let's find out.
I looked up the list of editions for a variety of rpg games that have been mainstays of the hobby and this is what I found. Note that these lists only include direct descendants and not spin-off games such as Basic D&D or Call of Cthulhu D20.
Call of Cthulhu
Chaosium: Call of Cthulhu 1 (1981)
Chaosium: Call of Cthulhu 2 (1983)
Chaosium: Call of Cthulhu 3 (1986)
Chaosium: Call of Cthulhu 4 (1989)
Chaosium: Call of Cthulhu 5 (1992)
Chaosium: Call of Cthulhu 5.5 (1998)
Chaosium: Call of Cthulhu 5.6 (1999)
Chaosium: Call of Cthulhu 6 (2004)
D&D
TSR: D&D (1973)
TSR: AD&D 1 (1978)
TSR: AD&D 2 (1989)
WOTC: D&D 3 (2000)
WOTC: D&D 3.5 (2003)
WOTC: D&D 4 (2008)
GURPS
Steve Jackson Games: GURPS 1 (1985)
Steve Jackson Games: GURPS 2 (1987)
Steve Jackson Games: GURPS 3 (1988)
Steve Jackson Games: GURPS 4 (2004)
HERO
Hero Games: HERO 1 (1981)
Hero Games: HERO 2 (1982)
Hero Games: HERO 3 (1984)
Hero Games / I.C.E.: HERO 4 (1989)
Hero Games: HERO 5 (2002)
RuneQuest
Chaosium: RuneQuest 1 (1978)
Chaosium: RuneQuest 2 (1981)
Avalon Hill: RuneQuest 3 (1984)
Mongoose Publishing: RuneQuest 4 (2006)
Shadowrun
FASA: Shadowrun 1 (1989)
FASA: Shadowrun 2 (1992)
FASA: Shadowrun 3 (1998)
FanPro: Shadowrun 4 (2005)
Traveller
GDW: Traveller (1977)
GDW: MegaTraveller (1987)
GDW: Traveller: TNE (1993)
Imperium Games: Traveller 4 (1996)
Mongoose Publishing: Traveller 5 (2008)
Vampire: The Masquerade
White Wolf Publishing: Vampire: The Masquerade 1 (1991)
White Wolf Publishing: Vampire: The Masquerade 2 (1992)
White Wolf Publishing: Vampire: The Masquerade 3 (1998)
Warhammer Fantasy Roleplay
Games Workshop: Warhammer Fantasy Roleplay 1 (1986)
Black Industries: Warhammer Fantasy Roleplay 2 (2005)
Here is what the average length between editions comes out to for each of these games. Formula is (2008 - year of 1st edition) / number of editions.
- Call of Cthulhu: 3.4 years
- Dungeons & Dragons: 5.8 years
- GURPS: 5.75 years
- HERO: 5.4 years
- RuneQuest: 7.5 years
- Shadowrun: 4.75 years
- Traveller: 6.2 years
- Vampire: 5.7 years
- WFRP: 11 years
It doesn't look like new versions of D&D come out more often than most of the other games out there. In fact, the ones that do better in this regard (Runequest, Traveller, and WFRP) mainly do so because the game was neglected for a number of years.

I don’t think an average is quite the right approach. The evolution of a game isn’t linear. If new editions are based on evolution of the design, then I would expect rapid evolution at first but a regularly slowing pace thereafter. It might be 1 year, 2 years, 5 years, 10 years, &c.
(Oddly enough, if you consider D&D3e a new/different game from AD&D; then the intervals between D&D editions starts looking a lot more natural. (^_^))
Of course, what a publisher decides to label a “new edition” doesn’t have to have anything to do with design evolution. In fact, what it means to be a “new edition” varies from game to game & even from edition to edition.
Oh, and “neglect” can sometimes mean “thankfully not messed with”. (^_^)
Tabletop RPGs are special in the fact that they’re timeless. If the game is fun, then just keep playing it.
If a new edition makes the game more fun, then it’s worth the money. If not, then just keep playing the older edition. It’s not like a computer where an old version just refuses to run with your OS or video card or something like that. Instead, a great classic game system is like an old friend. You can always go back and have a good time.
TSR: D&D (1973)
TSR: Greyhawk (1975)
TSR: AD&D 1 (1978)
YSR: Basic D&D (1978)
TSR: Basic + Expert (1980)
TSR: B,X,C,M,I (1983)
TSR: UA + WSG/DSG (~1985?)
TSR: AD&D 2 (1989)
TSR: Rules Cyclopedia/WotI (1992)
TSR: Player’s Option edition (1995)
WOTC: D&D 3 (2000)
WOTC: D&D 3.5 (2003)
WOTC: D&D 4 (2008)
2.69 years. That’s not counting the real variant systems like Dragonlance SAGA with the fate deck mechanics, or highly variant self-contained worldbooks, which are really just as much of an edition as many you’ve counted for other systems.
Still, WotC is far better on this than TSR ever was. They have the decency to put their experimental rules in d20Modern and Star Wars releases and leave the core alone for a while.
I see Basic D&D and D&D as being two separate game systems. They don’t use the same rules and are about as far apart as GURPS is from TFT. I also don’t see counting books of optional rules as being new editions either.