The longest I've had a maiden go over her due date was five weeks, and that was allowing for a 342 day pregnancy. The foal came out virtually fluorescent orange with those horrid pink-rimmed eyes, looked seriously undercooked, and actually gave the impression of having started to regress from 'maturity'. Thankfully, she did nothing but improve from the day she was born - strengthened, filled out from the origami character she was at birth, turned into a complete monkey, completely outliving her name of Mouse, and became a superstar in the show ring apparently.
I am waiting for a mare at the moment this is her 2nd foal, her 1st she went 324 days, she is currently at 319 days and is waxed up ready to go, so i think she is just an early bird.
Toffee was five days early. She also had hers at breakfast time after the evening of a really bad storm. I don't think she wanted to be pregnant a day later!
Maiden mare 367 days and this was 8 days after the mucous show - even though the books tell you tell you that they are certain to foal within 48 hours of the show. Vet said she must have been hanging on to it as we had a change to colder weather.
Forgot to mention that my maiden mare (mentioned earlier) probably didn't foal on time because that year, we had a really cold early May and we reckon that she held onto him until the weather improved - she was right, as the weather improved and we had the most wonderful warm and sunny couple of weeks! Maybe she knew a lot more than the books!!
A knowledgable friend told me : it'll come out when it's cooked!
My mare was inseminated in the UK and now living in Australia (with me) so summer has been switched around for her, poor soul. So we will see if this makes a difference.