from experiance it depends on the mare - I got the vet out over Christmas week one year as my mare was off her food and behaving badly etc - turned out she was just in season ! Which had never even crossed my mind !! (and cost me £40)
There is a period of low sexual activity in mares throughout the winter months. However, some mares can continue to come into season all winter, it is not that uncommon, though it is sometimes more irregular than than in the spring when the longer daylight hours stimulate part of the brain and the mare comes into season on a more regular cycle.
If you are finding that your mare comes into season all the time it is likely that she has an infection and may need antibiotics, again this is not uncommon.
I don't generally notice when my mare is in season, but last weekend i went hacking with a friend on a gelding she didn't know and hey presto, in season, really excitable and has stayed like this all week. Like you i was surprised that she would still be in season now.
Afew people ive asked this week have said their mares often have a big season as their last one before winter....and it is often at this time of year, just as it turns quite abit cooler.
the mare in question is new to me so dont really know her yet didnt notice she was in season till last night when a gelding was tied up outside next stable all i can say is it was pornographic has anyone used the variouse mare supplements with success