I am currently "laptopless", with a Linux-based netbook and my iPhone serving my mobile computing needs, but I don't count the netbook as a "laptop" because I use it so infrequently. My wife's MacBook died, and I realized that I rarely used my MacBook Pro, so I gave her the Pro.
Before that, here are my prior four laptops:
PowerBook 5300c, given to me in 1998 while I was in college.
Sony PictureBook C1X, one of the first "netbooks", especially when I added WiFi, purchased in very late 1999.
12" Aluminum PowerBook G4, purchased new in Summer 2003.
15" MacBook Pro, ordered on day of release of the Intel switch, January 2006.
I'll probably be getting an 11" Air soon, as I find myself wanting more than the netbook, but less than the MBP. I miss the size of the PictureBook (9.8" widescreen, the keyboard was about the smallest my fingers fit,) as well as the PowerBook G4. I was disappointed at the release of the MacBook Pro that there weren't any smaller models; but my PowerBook was getting very long in the tooth, and starting to have strange failures (later diagnosed as a failing hard drive.)
So, not counting the short span between the used PowerBook 5300 and the PictureBook, I went 4 years then 3 years, now I'm up to almost 5 years; for an average of 4 years.
My desktop computers last *FAR* less time.