All drive sectors have a certain amount of writes in them before they will no longer properly hold a charge, but it usually takes years to reach such a point where it becomes an issue. Basically if you scan the hard drive and the number of bad sectors is constantly increasing then it's time to consider replacing the hard drive before important files are lost.
That being said, depending what type of formatting you are doing, you may not really be overwriting much of anything. If you choose the "quick format" option then you are only overwriting the file pointers, the files/folders are still there only the pointer to the space has been marked as "free".