Since few time I observed that whenever I make french fries, after 15-20 mins of its making it is getting loose. Is there anything doing that we can maintain the crisp of the fries for longer span i.e. for at least 20 mins or so?  
The tip I hear the most with fries is, to double-fry them. First fry them till they are cooked, but still pale on the outside. Then let them cool off. Then fry them again, till they are crispy.  This should improve overall crispiness, but also potentially keep sem good for longer, as you´ll reduce water content.  That said, fries are greatest when they are hot. So try to make smaller portions and then make some more if you need them. With the above tip it´s easy, as you can have them prepared and the second fry does not take long. 