To paraphrase what Carl said - if it ain't broke, don't fix it. However, I've heard of folks getting a full set of new grids for less than buying 5 grids 'a-la-carte' - if you can find such a deal, you'd have back up grids for the future.

The expected lifetime on them depends on how well they are treated but the worst thing for them is being 'over driven' (running the filter with too much pressure) which is usually the result of not backwashing in a timely manner or of overloading the DE. Have you seen this thread?

(thanks Mom )