Probably about 3 years or so. Although, I did see some older boards like the 990FX Sabertooth and the X58 Sabertooth get NVMe support later in life via a bios update.
My x99 boards (1x Asrock, the other an Asus) got updates after the spectre & meltdowns were first announced. I was surprised as anyone else. Didn't think they would bother being so far out of warranty.
Just updated some 2011 Optiplexes with an update release a few months ago. Other than that, I've never seen more than a year or two after release for consumer pre-builts and three or four years for DIY board.