In case of intraday orders, you have to sell the stocks before the end of trading hours. If you do not, your stocks will be squared off at the end of the day.
In case of delivery orders, you can sell them from the day you purchase them. Note: In case of T2T stocks, you will have to wait till they get delivered to your demat account. 
To know what a T2T stock is, please press the button below.