“How long should you nurse a baby at a time?”

It is more important to have good sucks and active feeding sessions than lengthy feedings. At that age baby is just learning to feed, remember and these are all part of the practice.
Another thing is to remember not every session is a sucking for food, sometimes baby maybe thirsty and not really hungry so those sessions maybe shorter.
But as a guide the first week of life around 15 min one side and at least as much on the other if you have average milk supply can be used as a good gauge