Generally speaking, how long does a good laptop last? I've had mine for 5 years........minor little issues but otherwise works OK.
As with all technology, there are two main factors that will determine how long it will be of value to you: how well you maintain it and what you want to do with it.
If you take good care of a laptop, there's no reason why the physical hardware won't last for 8 to 10 years. The most likely device to mechanically fail during that period would be the hard drive, so making sure you have a solid backup system is always a good idea.
If the laptop is making any strange noises, takes an eternity to startup or experiences strange errors at random times, you'll want to have it examined to best determine whether you should repair it, upgrade it or replace it.
If the physical components all seem to be in good working order, then the next criteria would be what you need the laptop to do for you.
If you decided that you wanted to start exploring high-resolution photo editing, video editing or hardcore first-person gaming with it, I would immediately tell you to start looking for a replacement.
If you told me that all you do is check e-mail, casually surf the Net, write an occasional letter and do some online banking, then there's probably no reason why you couldn't keep using it until your needs changed or it required an overly expensive repair.
If you are running an older operating system such as Windows XP, that would be a bit of a concern if you're on the Internet. Windows XP was first released in 2001 when the Internet wasn't such a dangerous place and even with all of its patches and updates you're taking a knife into a gun fight when it comes to today's sophisticated threats.
More importantly, Microsoft will end all support of Windows XP in April of 2014 which means if a major security issue is discovered after that, there will be never be a patch to protect you.
My advice would be to either upgrade to Windows 7 (yes, it's still available!) or replace the unit by April of next year.
If you really like the unit and want to extend the life, doubling the RAM ($30-$75) will generally give it a nice little increase in performance, but if you really want to kick it up, you could replace the hard drive with a high-speed hybrid hard drive (starting at $149 installed).
This would both increase the performance and pro-actively replace "the most likely to fail" component in your laptop.
I know that the radical difference in Windows 8 is causing a lot of people to want to hang on to their older systems as long as they can, but that may not be in your best interest.
At some point, you won't have a choice, but for the time being, you can still get laptops built with Windows 7 or have special software added to a Windows 8 system so it looks and feels like older versions of Windows.
Always remember that YOUR needs will always determine when something is obsolete, not some random industry standard or the needs of your nerdy friends. Make sure you get with someone that can help you properly evaluate what you currently have, what you currently need to do with it and what you might want to do in the near future to make the best long-term decision.