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Bought mine in 2007 and intend to keep it for at least another 3 years. It feels like its new.
 
Personally I don't think anyone's questions are 'useless'. But I'll bet you can get 5 years out of your new computer, maybe not much more if you really want system software updates. I had a Titanium that worked great for almost 8 years, but the lid broke and at this point I really needed to be able to update to the latest system software. But figure at least 3 years for general use, and more if you don't mind knowing that at some point this new machine might not be able to take new system updates as Apple continues to evolve. Good luck.
 
I got a little over 2 1/2 years from my Powerbook G4, and if I didn't find such a deal on the MacBook that replaced it I'd probably still be using it. Considering I do more programming now than Photoshop I expect to get 3 or more out of my new MBP.
 
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