It really depends on how much information moves around in your computer. The point of defraging is to reorder your files in a logical manner. An anology that is often used is a parking lot.
Imagine in the apartment complex where you live everyone has a car and most all of the spaces are full but there aren't any assigned spaces. Throughout the day as people go to work, go shopping,whatever, they leave and return. Where they were parked at the beginning of the day and where they end up at the end varies depending on the best available spots.
Some cars will always be in the same area because its closest to their apartment, but for the most part, if a car leaves, it will not return to the same spot. So if you want to know how often you need to defrag, you need to consider how many cars you have in the lot (how much memory you are using), how many spaces are in the lot (how big your harddrive is), and how often do cars leave the lot (how often do you delete files). Nowadays, 2-300 gig hard drives are common.
The laptop i'm using has 232.2 gigs of usable harddrive space. I'm using less than 80 gb of it, and most of that is Windows Vista. My recycling bin has less than 2 gigs of info in it.So i'm only using roughly a 1/3 of my parking lot, and in that parking lot only 2.5% of the cars have been moving around.
So needless to say I don't defrag often. But if I still had my desktop from '00 with it's 20 gig harddrive.2 gigs in the recycling bin, ready to go out the door, That's a 10% change, and i'm more likely to move files around and delete files for space.So I would need to keep my comp defragged to keep it running smoothly. So my advice to you, If you are using less than 50% of your HD space, and delete less than 5% worth a month, once every 3 months is fine.
But if you delete a lot of files often (hold them in your recycling bin before you defrag so you can get a good idea when to defrag) then go ahead, do it as often as you want. Defragging won't hurt your computer, it just takes up your time. BUt you won't see a significany improvement, unless you've deleted a significant amount of files.
How often depends on the rate at which fragmentation occurs (which can be anything from once a week, month and for some daily). An easy way to not have to bother about defrag is install an automatic defragger and set it on auto. It will defrag whenever needed and latest defrag technology also prevents fragmentation from occuring.
In my personal experience A moderate user (uses his computer on a regular basis for Internet, email, word processing, or other productivity tasks,Digital photography activity, music downloading, and frequently adding and removing programs) should defragment their hard drive not more than once every six months. An advanced user ( who uses his computer for several hours each day for intense activities like high-end video gaming, video- or audio-editing, and advanced photo manipulation) should defragment their hard drive(s) once every three or four months. What I found helpful was a Free Third-Party Defrag Program is Easy to Run.
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