Well... in Lancaster I knew a lot of people did it in November. It depends where you are and stuff, but I'd advise leaving it later rather than earlier. It gives you more time to get to know people and to decide who you want to live with! You don't want to commit to living with people you don't want to live with
It varies across the country - in manchester people look in jan / feb (although I know one group who did it in dec!) but I believe in London the househunting starts in August.
Best tactic, if you're worried, is to ask someone you get along well with if they want to live together, and go from there - there's likely to be other people, and it's surprisingly common to live with friends of friends - who of course them become your own friends.
Me and the guys I live with started looking before the Christmas holiday (primarily because I tend to take charge of things and make sure they happen) and then sorted it when we returned. Most people didn't start looking until Jan/Feb at the earliest.
If you really can't find people to go with the university should have a system for putting you with others in a similar situation or finding groups that don't have enough, etc.
Between december and march is the norm. Don't worry, everyone feels like that
I have been to two universities and at one we did it all in Jan/Feb and at my most recent one we did it in November/December (however this was purely because we already knew who we wanted to live with and had placement after xmas so decided it was easier to get it all sorted in the first term. People were still looking for houses by Easter!)
Started browsing from January, usually had everything sorted by March/April. People usually think you have to find a house ASAP but there will always be somewhere available; I have a few friends who still haven't signed a place for next academic year. However, obviously, the earlier the better (in a non-contradictory way).
I did mine early January, but know people who sorted it in November as well as people who literally only just sorted it in the last few weeks or even people who are still looking. It's the kind of thing that just works out somehow or other.