The answer to this may well be regional, so you might get differing opinions form different parts of the country.....
As far as I'm concerned, and I'm talking about the south of the country (Reading, Berks and surrounding areas), AND I'm also talking primarily about still waters, I tend to stop using them in December BUT again the weather and more specifically water temperature, will also have affect my decision. If we have a reasonable December and this continues in January (without major drops in temperature) I have still caught roach on them.
What I have written above contains a lot of 'if's and but's' but this is how it has been for me over many years, and when I look back through my basic diaries I haven't caught a great deal after Christmas on tares. What I will say is that if I have stuck to certain swims on certain still waters, and fished them several times a week with this bait, feeding with hemp, I am quite sure you can prolong the period roach will take them.
The same goes for hemp; Some years I cannot get roach to feed on it (more or less) after the end of November, but then I have had caught 40+ roach on the stuff on 22nd December!! Again I'm talking still waters).
Personally, I think it never a bad idea to carry a small bag of tares and hemp taken from the freezer which might just the chance of a fish when other baits fail as we move into the cooler months. If they don't work pop them back in the freezer........
Water temperature, air temperature, wind direction, etc. all play a part, so taking these factors into account, I would suggest this; If you fish for roach and have swims that produce good fish regularly, then concentrate on them and stay with them throughout Oct/Nov/Dec using the tares with hemp feed. Give them a chance to work and not just try them for half an hour!!
Hawb811...............Apologies if you are already aware of the above. It is alos just my experiences with this bait and others might offer different opinions.......
Good luck, though!