Not to be a smart ass, but whenever it wants to be. Actually I'm sure if you called CDOT or the State Hwy Patrol they could tell you.
Around here the County, BLM & USFS stop maintaining the roads by about the first of November, as they depend a lot on part time summer help. In the case of the feds, their help is gone by early Oct. My neighbor does road work for them and has every winter off. Also around here they lockup a lot of the FS roads once winter gets here, too much damage from wheeled vehicles when they are muddy.