I'll place my vote after writing this: 10+ years.
Back around 1983/1984, I took Grandma's old Samsonite over-nite suitcase (a little bigger than a portable electric typewriter), and built a "mobile" computer using a Timex-Sinclair 1000 (Z80A Zilog processor). It had a rechargeable battery, which fed the Sinclair, a small thermal-printer, and a Sony Watchman TV (the monitor). I was able to "do computing" for the length of a ride on the Washington Metro from Clarendon Station to L'Enfant Plaza. It was a good "demonstration" item, but not practical. Dealing with the battery-pack and the short charge life powering those three items -- it was too much trouble. I backed away from the emerging laptop technology after that, because of the battery issue and lack of a real need. I didn't travel enough to need one. And fact is, wireless technology emerged to a practical level after the earlier IBM thinkpads.
I put off acquiring any laptop device during the last decade for similar reasons. Now, I've acquired a six-year-old refurbed/surplus Gateway E-475M. I upgraded the HDD to SSD, and replaced the wireless network a/g card with a wireless-n model. I also upped the memory from 2GB SO-DIMM DDR2-667 to 4GB DDR2-800. The processor is a Centrino C2D T8300. [Apparently, someone had replaced a less-powerful stock T7xxx with the T8300; the model didn't ship with anything more than the T7xxx as far as I know.]
The laptop cost me about $250+ and the extras brought the total to under $500.
I can't say how long this will last for me. I'm rocking an i7-2600K in my desktop system right now, and have no inclination to upgrade or update that item for a while. I'd been in the habit of building a machine for myself about every two or three years, and was building systems for the extended family so that my projects averaged about once every two years.
The thing I like about this 8.5 lb "executive class" lappie is the charge-life of the battery -- about 4 to 5 hours. When it loses its charge potential and needs replacement, I can get a 12-cell unit that will likely last all day. Looking at retail offerings in the weekly Frye's ads, for more than half my expenditure on this unit, I might have had an Acer notebook -- dual-core i3 -- with a battery-charge-life of less than 2 hours.
And buying a new machine with a warranty or even a service agreement -- you would be reticent to grab a Phillips screwdriver and start fiddling with the hardware. I didn't hesitate to do that right away with this old Gateway. It was -- in fact -- a "learning" process which cost me some money. [It often does, anyway.]
I've opened up the wireless feature on my household router, and I'm getting darn good wireless-N speeds. I'm going to improve that some more -- and soon.
Yup! That's about all I can say. THIS . . . IS . . . MY LAPTOP. Now that I've put that behind me, I'll start looking for a tablet of some sort -- Android, iPad -- maybe a "Surface" -- and take my darn time before I shell out more buckets of ducats. Oh. I can upgrade my cell-phone to "smart."
"Beam me up, Scotty!" Whoop-i-ty doo-dah! [Big deal . . . ]