Sorry for this generic answer but it is the truth.
When the snow gets deep enough to cover their feed up high to where they can''t get to it. It might start in October or December, it just depends on the year. I know of a herd in Utah that winters at 9500' + just because the wind blows the snow off of the high ridge that they live on and there is feed all winter long.
Like this last winter for example. Where I live here the winter was so mild I never saw any elk come down as usual. In general there are several hundred not far from the house by December/January but they never made it all this year.
If you're like me and use a plastic tote to store your gear in then you might want to read further. When plastic totes are manufactured they are typically shipped with the lid on. This makes the inside of the tote smell like the plastic it's made of. To remove the plastic smell from the tote I use baking soda. I open the baking soda up and leave the box in the tote with the lid on for about a month to remove the plastic smell. I tried just leaving the lid off but I was still able to smell...