I have 3 really nice high end AR's that I have never fired and are brand new factory rifles.
The oldest is an LWRCi that I bought new in Jan. of 2012, then a Daniel Defense I purchased in July of 2012, and lastly a Bravo Company that I bought from a member here in Sept. of 2015. He bought it new and never fired it, I bought it from him and have yet to fire it.
It is on my agenda for 2018 to get them out and at least run a few mags through them too make sure they all run, don't see why they wouldn't, but you never know.
So to answer the question, as of now, 5 years.
dang... they're begging for some action