Personally I would leave it until you're settled in, this way you can get an idea of what hours you'll be able to work without it interfering with your studies and/or leaving you with no social life. You'll also know the area better so are less likely to apply somewhere then find you can't get there on time or it's in a dodgy area with no late buses etc. And the last thing you want to be doing is excluding yourself from Freshers' events / other stuff going on when everyone is still getting to know each other because you have to work.
I personally left it until February, though that was because in my previous job I had always been given loads of hours and found it difficult to get time off when I wanted it and I wanted one Christmas and my 21st weekend work-free. If it hadn't been for that I'd have probably started applying around November.