Usually a generation lasted about 9 years. (pull it up) Due to a rapid increase in technology, the ps2/xbox/gamecube had an extremely short lifespan of 5 1/2 years. I think they will not mention 8th gen consoles until a year after 3DS (2012), and the demo will be 2013, and they will come out late 2013/2014. Really I say 2014 though.
No . . . they've been around 5-6 years for a while, if anything, the space in between systems has been steadily getting longer since the start of the industry:
Playstation: 1994
Playstation 2: 2000 (6 years later)
Playstation 3: 2006 (6 years later)
Nintendo Color TV: 1977 (JPN only)
Famciom/NES: 1983/1985 (6 years later)
SFC/SNES: 1990/1991 (7/6 years later)
N64: 1996 (5 years later)
Gamecube: 2001 (5 years later)
Wii: 2006 (5 years later)
SG-1000: 1983
Master System: 1985 (2 years later)
Genesis: 1989 (4 years later)
Saturn: 1994 (5 years later)
Dreamcast: 1998 (4 years later)
XBox: 2001
XBox360: 2005 (4 years later)
Atari 2600: 1977
Atari 5200: 1982 (5 years later)
Atari 7800: 1986 (4 years later)
Atari Jaguar: 1993 (7 years later)
Neo Geo: 1990
Neo Geo CD: 1994 (4 years later)
Game Boy: 1989
Super Game Boy (SNES cartridge which could add allow color pre-GBC): 1994 (5 years later)
Game Boy Color: 1998 (4 years later)
Game Boy Advance: 2001 (3 years later)
Nintendo DS: 2004 (3 years later)
Nintendo 3DS: 2011 (7 years later)
And the third generation was from 1983-1989, not 1980-1989 as Wikipedia states on it's system list (it even says 1983 when you click on the third generation), the video game industry tanked in 1982-1983, making a noticable divide.