The hotel rooms usually begin getting booked in the Winter, so the section allotted to Kumoricon usually gets booked up by mid-Spring. This is an estimate, though, as the room block gets expanded several times, generally. By this time, I'd say your chances of finding a room at the hotel are pretty slim, though. Your best bet (what lots of people do) would be to either look to room with people who already have a room there, or look at other hotels in the area.
So, in short, the rooms usually get filled up by Spring. The best thing to do is book your room first thing of the year, if not earlier.