Sheikh Muhammad Muhammad Sâlim `Abd al-Wadûd
Allah says: “And the mothers should breastfeed their children for two whole years for him who desires to make complete the time of nursing.” [Sûrah al-Baqarah: 233]
This verse makes it clear that the complete duration of the nursing period is two full years.
This means that no mother can be obliged to nurse her child for longer than that. Also, it means that the father is not obliged in Islam, to pay for a wet nurse for more than two years. Also, after the two year period is over, a father who is divorced from his child's mother will no longer have to provide the specific support that the divorced mother is entitled to receive for nursing the child.
At the same time, we do not know of any evidence to prohibit a mother from nursing her child for more than two years if that is what she determines to be in the child's best interests.
And Allah knows best.
Source: Islam Today