I was told by a key helmet designer who has worked for three major cycle helmet brands that they simply cannot put a figure on the timescale.
Three major things affect the helmet, water (sweat and rain) UV Rays, and impacts.
One end of the usage spectrum is a helmet stored in a dark and dry garage, placed carefully onto head and then back on a shelf after use, used once a month, will last for many years.
Other end of the spectrum, racer guy rides every day, does loads of miles, hammers his (or her) kit, slings the helmet in the back of the car and leaves it there all day in the sun, bashes it about and never washes the sweat off it. The helmets will in theory only last a few months. But he will Probably buy anew one, or smash it and replace it a few times a year anyway