The GameBoy/Portable line has always took longer... or shorter.. Well, the portables have never had a set schedule.
Only counting platform changes:
The original GB came out in 1989. (Includes pocket and colors)
The replacement was the Color which came out 9 years later in 1998.
Its replacement the GBA was in 2001. (Includes GBA SP and micro)
The DS was in 2004. (Includes DS lite, DSi and DSi XL)
The 3DS will be when? 2011? 2012?
So 9, 3, 3, 7/8 year gaps between architectures/platforms. Or devices that actually changed the device enough to require upgrading to play newer games. Though I guess you could also count the DSi if that's right since some games do require the DSi, right?