When the new shiny MacBook comes out.
Just kidding, I try to get the most out of my machines, my current Macbook has lasted 4 years so far and is running strong.
As suggested by @Ivo maybe see how you could extend the life of your current machine; putting an SSD in gives a huge improvement, I installed one in my computer last year and it boosted its speed substantially. If you have 2 sticks of 2gb ram, see if your machine can support 4x4. Also, a clean install of windows 10 always gives a nice boost by getting rid of things that are running but aren't potentially needed.
However, 7 years does sound like a long time. Maybe it is time to buy ;)
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