The average gestation period (or length of time a cow is pregnant) is 285 days or a little over 9 months.
Depending on breed and nutritional status of a cow, the actual time a cow is pregnant can differ; some will be shorter (~270 days), others longer (295+ days).
The Bovine Practicum Q&A "blog" is an informative, just-for-fun section where I find a variety of questions that are often (and not-so-often) asked by inquisitive people like you, and answer them to the best of my ability. Much of the questions have come from a wide variety of reaches from the Internet.
Popular topics include: Cows, feeding, breeding, beef, slaughter, calves, dairy, milking, breeds, grazing, bulls, feeds, and much more!