My wife's pc is a i5-3570k with a gtx660ti. That's still going to be good enough for the next 5 years, even though it's already 4 years old. How long will it last is a very almalgamous question. The answer is only something that's defined as being relevant to you, Op. For all we know, you might play the same few games for the next 5 years, never really getting into anything new and cutting edge. In which case the pc is good for at least the next 5 years, as long as the parts hold up. You might decide it's definitely time to go with OC in 3 years, in which case the Z170 board will be useless since Intel will have released a new cpu that's not lga1151 by then. It might be that you are the kind of person who likes cutting edge gaming at maximum with a 4k monitor, in which case your build was obsolete yesterday.
Unless your tastes change, that build will easily last 5 years. How relevant it will be in 5 years time is impossible to predict.