There is no exact rule on how often since only you can check how badly and how fast your drive gets fragmented. If you do a lot of adding, deleting, downloading files often, its likely that the drive will get fragmented pretty fast. Once it starts, it proliferates subsequently like a disease if its not checked. For a while you could analyze the drive periodically to make out the pattern. If its a bother to keep checking and doing it manually, opt for one of the automatic tools.
