The main reason a window hinge or friction stay needs replacing is when the black or grey friction block on the window hinge breaks in two. Which will prevent it from opening and closing correctly. Other reasons window hinges or friction stays need replacing ;
1) when you close your window you can feel a draught along or down the hinge side of the window
2) when you close your window the window does not pull in at the top or down the side of the window.
3) A rivet may have broken from the window hinge leaving the window unable to be closed.
4) with larger windows the hinges can bend and buckle when you try to close the window leaving the window impossible to close.
Please note that although your window hinges seem to work properly if you are feeling draughts through the window when it is in the closed position then it is almost certainly the window hinges that are worn. A good test is to use a credit card , if the card can slide easily between the opening vent and the rubber gasket on the outer frame the window hinges are almost certainly worn.