I have been out there many times over the years. Probably Mid to late February is the most consistent, but that is not saying much. We no longer will book trips out there months in advance because the conditions vary so much. So, now we watch the conditions and just book a week or 2 in advance. There are a lot of landmines out there offtrail (rocks and stumps) so, they need several feet of snow on the ground to avoid hitting these. (we have damaged a number of sleds over the years including 2 new ones that were totaled) I don't think they had enough snow at anytime this winter for good offtrail riding.
If you are looking for reliable deep snow and a true mountain riding experience, I would also recommend going further west. Our group refers to the Black Hills as kindergarten mountain riding. That being said, they have about 350 miles of trail loops that are groomed almost every night, and are very scenic. It's a great place for less skilled riders, kids, wives, etc. to learn some mountain riding skills, and the offtrail meadows, valleys, and hills out there can be a lot of fun when the snow is sufficient. Plus, very minimal risk of avalanches.