I have a late 2011 MBP, I hope to use it another 2-3 years. I think it might die before it's "outdated" due to the annoying AMD GPU issue (which I haven't had a problem with so far, knock on wood). We'll see how it goes, but it's been going strong since I got it. I threw a 512 GB Samsung SSD in it and updated the RAM to 16 GB. It's very fast and handles everything I need very well.
I guess the big reason I'd want to update sooner than later would be to get the Retina screen that are in the new MBP's, but I'm good for now.