Let's say I've just bought a new laptop and it is under warranty. I assume it means a free cleaning when it is due. Let's say it's even extended warranty. Let's say I have an older laptop and it's not under warranty anymore, so paying for cleaning is up to me. Let's say I'm a tinkerer or a Dell service technician trainee and I want to practice clean the fans of my laptop - an older model, good for such purposes - myself. Let's say I'm just anyone caring about the health of her notebook. I understand recommended cleaning intervals may vary depending on your circumstances and laptop makes, but what's a good general rule of thumb, what does the doctor prescribe? Which Dell service manual deals with best practices on how to clean laptop fans, in general?
I would recommend cleaning the system as needed depending on the environment. If the system is in a dusty environment maybe more often, but it all depends on if you can see dust near the fan vents. As far as manuals, I would recommend going to this support link and entering your Service Tag to get information on how to easily access the fan using the Service Manual. Make sure to use compressed air and I would also recommend holding the fan while using it. There should be some good guides online at tech sites.
In my experience I found desktops have to be cleaned more often vs notebooks, but that might also be because on the desktops I own they have multiple fans vs the one fan that is typically found in notebooks.
I hope this helps!