EDIT:
@Sanu: I agree yaar that everything happened within a span of 24 hours but trust me had they dragged the apology bit and stretched it more, it would have been disastrous to watch. Nupur forgiving him so soon was true to her character sketch because she is not someone who holds grudges and takes things a little too far by refusing to hear what the other person has to say like Mayank does. Besides, Mayank's apology was worth forgiving, no person with a heart would be able to remain mad at him for so long after that earnest , heartfelt apology.