In the past i5's and i7's have been viable for a good 4-6yrs depending on needs, I wouldn't expect anything different from the current i7's than those from the 4th, 3rd or 2nd gen. Ryzen is due out in the somewhat near future but no real word yet on real world performance or pricing.
There's no ryzen available that's been pitted against existing cpu's across a variety of games or other applications to properly place it's price/performance. It's really no different than intel, it will mean a new cpu, a different motherboard, a reinstall of windows, ddr4 ram. Everything you'd upgrade to run intel you'd be doing for ryzen, at least intel is a known product in terms of price and performance.
Physically cpu's last for years, 7, 8, 10yrs...