It depends.
The bump stop keeps the shock from slamming into itself under full compression. Depending on the roads you drive and how you drive, you may rarely use the bump stop or you might use them quite often.
It is not unheard of for a stock suspension to hit the bump stop under hard cornering. If your car is lowered, then its even more likely.
The typical symptoms of worn out bump stops are a feeling like the car is bottoming out, harsh knocks when going over bumps, strange handling in hard corners, and at the extreme - suspension damage.
If you drive the car easy on mostly smooth roads, then you can probably wait until the shocks need replacement. If you drive on rough roads or like to push the handling, you may want to replace them sooner than later.
The biggest risk to the suspension is hitting a big pothole at 85mph. Avoid doing this if possible.
