How long should a new born baby sleep for at a time?

About 4 hours. Rule of thumb is that they should feed 2-3 ounces every 2-3 hours for the first few weeks of life and should be awakened for feedings if they are not waking up on their own. This ensures that they are getting enough calories each day.
About 4 hours. Infants or adults seldom sleep more than 4 hrs continuously. Infants may alert to feed, adults roll over & don't remember. After an initial 4 hour deep sleep an infant is capable of waking every 45-90 minutes. Feeding them at least 30 min prior to sleep separates feeding from their sleep ritual & decreases their demand to et every tie they need to go back to sleep.