Everybody's combination of fan hardware and print settings is different. Unless someone else has the exact same printer and slicer profiles as you, there's no way to really say anything like "use X% for PLA" or whatever. For practical purposes, you just empirically figure it out with test prints based on a few simple rules of thumb:

• Use lots of cooling for PLA, moderate cooling for PETG, and minimal cooling for ABS. (Note that sometimes ABS does benefit from gentle airflow, and PLA shouldn't necessarily always just be blasted at max power.)


• Use less cooling on big prints where each layer takes a long time, more cooling on small prints where each layer is very quick.


• Axial / box fan? You'll probably want to run it at full power. Radial / squirrel cage blower? You may want to run lower power.


• Overhangs curling up or sagging? More airflow may be needed. (Lower layer heights also help enormously.)


• Hot end temp sagging when the fan kicks on? Try less airflow. (Or insulate your hot block better.)


• Weak layer bonding? Try less airflow. (Or raise your hot end temp.)


• Small, fast prints getting mushy or corners "pulling in"? You need more airflow. (Or lower heatbed temps.)


• Print warping / corners lifting off the bed? Try less airflow, particularly on the lowest few layers. (Or higher heatbed temps, or a better adhesion layer, or less nozzle/bed gap for the first layer, or any other approach.)


• Fan too noisy? Try less airflow. (Or get a better fan.)


• Filament stringing during travel moves? Try less airflow pointed directly at the nozzle. (Or tune your retraction settings better, or get the moisture out of your filament, or lower hot end temp.)