It's called "ferberizing". The Ferber method is a technique invented by Dr Richard Ferber to solve infant sleep problems. It involves "baby training" children by allowing a child to cry for sometime before offering comfort.
Know More: There is no fixed "ferberizing" time suggested; but in the Indian context, if you can ensure that your baby is otherwise safe, at least 15 minutes should be good to begin with - starting at 3 to 4 months. Remember, your watch starts ticking from the time he actually begins to cry, not from when you laid him down to sleep. And start early; don't delay beyond the 5th month!
What our columnist Dr Shelja Sen says: The practice needs to be thought through carefully by parents before it can be implemented for their children. It also calls for tremendous discipline and self control on the parents' part.