There is a figure above which the drive itself will report a health problem in its overall SMART status but that will vary by drive and manufacturer. All drives have a number of sectors put aside ready to be used for reallocation as it is common to have faults on the drive medium as a result of the manufacturing process.
Reallocations themselves are not a sign of impending doom (in themselves they're a poor indication of drive health), but if you read through Google's statistics on hard drive failures you will see that when a drive begins to relocate sectors there is a "significant statistical probability that the drive will fail sooner than one that is not exhibiting this behaviour."
In short, you should only worry if the number of reallocated sectors is increasing consistently or becomes very large, say over a thousand.
Reallocations themselves are not a sign of impending doom (in themselves they're a poor indication of drive health), but if you read through Google's statistics on hard drive failures you will see that when a drive begins to relocate sectors there is a "significant statistical probability that the drive will fail sooner than one that is not exhibiting this behaviour."
In short, you should only worry if the number of reallocated sectors is increasing consistently or becomes very large, say over a thousand.