We are going through divorce. Due to our financial circumstances we are planning to live together in the same house and sell after 3 years once our son has graduated from University. Is there a time limit for a Mesher order?
That seems rather an unorthodox arrangement. The point of divorce is that you no longer wish to be married or live together.
In answer to your specific question, a Mesher Order has trigger points for sale of the property - things like remarriage, by agreement etc - and a clear end date. This would usually be youngest child reaching the age of 18/21/end of first degree.