Mad people - far too early.
I actually think 1st December is far too early too but I know lots of people put theirs up then. They would start to drive me mad and I would be sick of them by the time Xmas actually came around. Ours go up anywhere between 12th and 18th December - the later the better for me.
The earliest I have seen decorations up was last year when we went to the Lake District for the October half term. We drove past a house that had all their decorations up inside (from what we could see from the outside anyway). Mick actually turned around and drove back as we didn't really believe it. We drove past this year to see if they were up but sadly not.
The latest I have seen anyone keep them up (or maybe they were up extra early, or never got taken down) was quite a few years ago when I went out to see a landlord to talk about their houses that they wanted to rent out. Lovely posh house and they took me through to the conservatory and there was their Christmas tree in all its glory, lights on and everything. It was mid April.