Cleaning a laptop fan can make a big difference to your laptop's temperatures, and let you enjoy a more serene laptop experience without the fan whirring away at you.
The most straightforward way to clean a laptop is with a can of compressed aid.
You also can use a vacuum cleaner to suck material out of your laptop.
Cleaning a laptop fan is almost entirely safe if you don't take the laptop apart.
If you're comfortable with opening up your laptop, then by all means, go ahead and do so.
Everybody's habits and environments are different, and so how often you need to clean your laptop fan depends entirely on your circumstances.
If you work in a relatively clean office or home environment, then you can probably get away with cleaning your fan a few times a year.
If you work in a construction zone, or some other very dirty or dusty locale, then you could benefit from cleaning your laptop fan once a week.
Certainly, if your laptop starts performing more slowly, dirty fans could be a contributing factor.