So you should clean your fan every 6 months to counteract this to increase the life of your fan.

If dust is caught in the ballbaring it will slowly trash your fan. The more air is being moved, the more dust will collect on the fan. More airflow means more prone to dust, and dust is bad.

It does depend on how often you buy a new pc, as a manufacturer usually has like 5 years of life expectency in mind, which can become like 4 or 3 years if you constantly run it at full. Although a fan is tested for a prolonged duration at maximum speed, the manufacturer expects the fan to not always be on nor being at maximum speed.

The faster the fan turns, the more resistence it has to endure on both the ballbaring and the motor behind it. If the noise is not distracting or discomfortable, its not an issue. For example, outside of your computer case. Make sure that the air that is being moved can safely move to an area where it can become hot without a problem. It moves air faster from one area to another. Running your fan at maximum speed does a few things.

Good fan speed for pc