That depends on what you want to do in the future.
If you just want to run the same software you run today, it could last longer than you.
I generally don't feel the "need" to upgrade until I want to something with my PC that I either can't do, or that my PC takes too long to do.
3 or 4 years ago, I tried to flip a 1080p video and my old single-core rig needed 3 months to process ~15 minutes of video.
It was then that I decided that I "needed" to upgrade.
When Crysis 3 came out and I couldn't play on max settings, I only "wanted" to upgrade. It wasn't until the price dropped on the 780 that I decided to upgrade. (again)
So the longevity of your rig depends on your expectations of performance.