You must regularly check your window grids for wear and tear to ensure that you don't need to replace them. There is no set rule regarding the longevity of these inserts. Since they are frequently exposed to the elements, they degenerate over time. Faded grids could be a result of direct exposure to bright sunlight and may not necessarily indicate wear and tear.
Check the strength of the grid. If it feels wobbly and worn, replace it. Another good indicator is the state of the connector holding the insert in place. If the connector looks old and about to crumble, change the grid immediately.