[Stooky Bill]

Wikipedia suggests they film for eleven months of the year, three months in advance.

For comparison EastEnders is usually six weeks ahead, although by this point in the year they've ramped up production to Christmas, and by the time Christmas does comes round they'll be complete to the end of February.

Apparently Doctors will stop filming in March and go off air in December. I don't think it  broadcasts over the summer usually, but next summer is the Olympics so they'll presumably only need 6 or 7 months in the can when they stop production