There is no standard length of time that you should keep your laptop before you buy a new one; it's a personal choice based on preference and perhaps, functionality. You can keep it for as long as its functional and you happy with it.
You can keep it as long as it works well. However, when the laptop cannot support the new operating systems and the one that is installed is no longer supported (Win95, WinNT, Win2000, etc) is a pretty good sign that is time to change.
You should at least keep the computer for about a good maybe if it keeps this long a good 3 to 4 yrs but it also depends if you are satisfied with what type of computer you have or if you computer is slow or even if your computer is broken in some way.
I have used my laptop for 3 years. It's still in good condition. I wanna buy iPad if it is broken someday. Nowadays the update of electronic items are quickly. Don't exchange so often or you'll regret.
it depends on 2 factors. 1. what do you have. if you have a high end notebook with high power processor (MPU), you can definitely run it for at least 4-6 yrs. if however, yours is a low end (low power MPU), 3-4 yrs may be max. 2. what do you do with your PC if you just internet surfing, email, then you can run for 4-6 yrs. if high power number crunching, video processing, gaming is your thing, then 3-4 yrs max. in general, you should be able to run your notebook for at least 3 yrs.