Not sure about how long it'll physically last for good computing (my HP is 8 year old and still going strong). But you'll be good for gaming and multitasking for atleast 2-4 years w/ your current specs (GTX 760 included). You might upgrade to i7 4790 after that if games require extremely high single core performance and multi threading, but that'll be atleast 4 years down the road.
In case of GPU, if you like ultra gaming, then you'd need to upgrade in 2-3 years, if you're ok with medium-high, then its enough for 3-4 years. RAM is sufficient for atleast 4 years.
All in all, you'd need to upgrade CPU, RAM, GPU after 4 years, especially the GPU. All are my best guesses