Looking back, I realised that the laptop was almost two years old. So, to answer the question in the title to this post, every six months to a year, probably. Don't leave it till you hear excessive fan noise, because the fan in the picture above has obviously been working very inefficiently for a long time before I noticed the noise.
In case anybody needs to know how to clean the fan on a Toshiba Satellite C50D-B-120, here's how to do it:
- Put the machine on its lid and take out all the screws in the back (you'll need a magnetic screwdriver to lift them out).
- Pull out the battery and the dummy CD/DVD drive (or the real drive, if you have a more expensive model).
- Lift up the case back by putting your fingers inside the CD/DVD drive housing and pulling up. The case back will pop up. Continue to pull up as the case back clicks open towards the other side of the case.
I cleaned the fan with an air duster. Stop the fan from spinning while you dust it to avoid damage. I also used a paint brush to clean the fan vanes because they were still dirty even after blasting with air, and then gave them another air dusting. Don't forget to clean the accumulated dust off the inside of the case back too.
Refitting is the reverse of the instructions above. The case back will need pressing into place round its edge.
Which leads me to the question: why do laptop manufacturers make it so difficult to clean a laptop fan when it is something that needs doing regularly?