Well, their gestation period is 9 months, just like a human. Her bag should get even bigger immediately before, but they don't always. Sometimes the milk doesn't come all the way down until right before. Have you checked to see if milk is present? This would mean (if due now) she conceived in late June/early July. She could be due in late April if she conceived in late July/early August, she would be due in April. It should be a approximately 252 days give or take. Remember, they are like humans, and each pregnancy can vary, especially if this is her first foal. To be very precise you could have a vet check; they will probably do a sonogram and get give you a due date accurate within a couple of days. A vet is also a great idea if you have never helped birth a foal. They can usually handle things by themselves, but you should be able to tell if there is a problem and when to call the vet. I know people that actually board their horse at the vets for him to be on call to deliver. Good Luck!
I am pretty sure that the gestation time for a horse is 11months and the average mare goes to about 340 days. I do agree that all mares are different. My mare was a 10 month foal and so was her sister according to there breeder.