Just for a more informed idea, I'll put up the release dates of consoles from the current makers to their successor.
NES > SNES - 7/6 years (Japan 1983-1990/US 1985-1991)
SNES > N64 - 6/5 years (Japan 1990-1996/US 1991-1996)
N64 > GC - 5 years (1996-2001)
GC > Wii - 5 years (2001-2006)
Wii > Wii2 - Should be about 5-6 years (2006-2011/12)
Playstation > PS2 - 6/5 years (Japan 1994-2000/US 1995-2000)
PS2 > PS3 - 6 years (2000-2006)
PS3 > PS4 - Should be about 6 years (2006-2012)
Xbox > Xbox 360 - 3/4 years (Japan 2002-2005/US 2001-2005)
Xbox 360 > Xbox 720 - Assuming Microsoft doesn't race out the door again, 5 years (2005-2010)
Those are pretty much my guesses based on the data we have. I'd be surprised if they were off by more than one year. As I already noted, I'd be surprised if we don't hear anything about the next console cycle at this year's E3 or TGS.