Question: How often do i need to do a disk defragmenter and/or a cleanup? 

Answer: The longer you leave it, the longer it'll take to defrag. If you do it once a week as people are recommending it'll be a quick and hassle free job. You can leave it longer quite safely but it's good advice. Also do the disc clean up BEFORE the defrag. There's no point wasting time defraging files you're going to be getting rid of anyway.
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