My experienced mares I don't have gestation lengths on, as they came from a pasture breeders place; and I haven't raised a foal out of either yet (both lost the foals they were carrying when I bought them; one at 24 hours old, the other late term abortion).
However, those that were maiden mares for me; went 320, 320, 307 and 324 (all B-size mares).