guys!! how do you know that you need to defrag your drive ?

1. it is base on how long you use your drive ??
a. execute defrag weekly, monthly, yearly
2. it is base on how big the amount of byte added to your drive ??
a. execute defrag in every (5GB, 10GB, 20GB) that added to your hard drive ??
before that let us know what is the purpose of defrag and you get the right answer above

Best Regards!!!