I usually clean my monitor every couple weeks, and I usually clean the inside every six months. I would recommend though, if you are in an area where dust is prevalent then you might do a internal cleaning every month.
Yes. But most of the time people don't think about dusting and it's really annoying when a friend asks you to help with their computer and you take the side off and heaps of dust flies at you... and one time a spider was in a case
Try to once a month