In the light of Quran and Hadith, how long a mother should nurse her baby daughter?
I have heard few people saying it is 2 years. And some people said 2 1/2 years.
What is, if any, harm in nursing after 2 years or later?
Is there any difference in nursing time for daughters and sons?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

In principle the breastfeeding period for a child, whether a male or female, is two whole years. Allah says: {The mothers shall give suck to their children for two whole years, (that is) for those (parents) who desire to complete the term of suckling…..} [Quran 2:233]

However, if the parents decide to increase or decrease this period without harming the child, then there is no harm in that.

Al-Qurtubi said when interpreting the above verse: “It contains an evidence that it is not obligatory to breastfeed for 2 years, because it is permissible to wean the baby before two years”. He added “increasing or decreasing the period should be with the consent of both parents and with the condition that the baby is not harmed.”

There is no difference between breast-feeding a baby boy or a baby girl, and there is no harm in breast-feeding them after the period of two years if there is no harm for the child or the mother, and as long as the husband does not object to that.

Allah knows best.