I ran through one of my BT's last winter (very mild) and the last time she was (audibly) in season was mid- January. I know because I got a new BT billy late January and would have mated her, but no she stopped.
But it all depends, my BT billy was born middle of July, so go figure...
January should still be safe, however she may go into cloudburst before that. If you are having a billy staying with you, you could always inject to bring her into season - your vet should know.