My own experience with Shiga Kogen early season having been going there for the past 4 years is that it generally is good enough by the 2nd to 3rd week of December to have a decent time of it. Watching the reports anxiously every season for snow fall as I was only able to take vacations during this period, quite stressful though. I've been lucky so far. This year I've managed to put off my trip until 2nd week of January.