you shouldnt do it for a long time. i would say do it for even less than a minute, because even though they hold up their head, it still hurts their neck and they dont know what to do. you really shouldnt do it until they can completely hold their head up on their own.
This is not accurate. The point of tummy time is to help strengthen their neck and trunk muscles, since babies are now back sleepers instead of tummy sleepers. They need to develop those muscles so they can learn how to roll over, sit and crawl. My pedi suggested as long of a period of time as your LO will tolerate - anywhere from a minute to 15 minutes. I usually flip my little guy back onto his back when he starts to fuss or cry, and try again for another couple minutes once he settles down.