I used to clean my case about once a month, and was growing concerned that, as time passed, the fan was running for longer and longer periods, and with less demanding games.
After inspecting the airflow while running the system, found out that the culprit was the GPU fan, which I hadn't noticed because it was covered by a big lid. Removing it showed me several years worth of dust around the heat sink, which was blocking most of the airflow. A good cleanup, and things were working as great as day one again!
