does a simple vacuum in the inside if my case do the job? and how regularly?
Get a can of compressed air to blow out the dust. I use the compressed air while holding a vacuum nozzle near the side of the case to capture as much of the dislodged dust as possible to prevent it from recycling back into the case.
How often depends on a variety of factors including how dusty your environment is. It helps if your computer has a clear side panel so you can more easily monitor the dust formation. I keep a flashlight next to my computer so I can examine all areas of the interior through the side panel.