Some interesting facts:
PS1, released Dec 94, 102 million sold
PS2, March 00, 120 million sold
PS3, Nov 06, 5.5 Million sold

XBox, Nov 01, 24 million sold
xbox 360, Nov 05, 13 million sold

If the trend continues,
Next xbox due somewhere in 2009 (2 years or less from NOW)
Next PS due somewhere in 2012 (5 years+)