depends...
With an AIO, "CPU fan" is actually the water block pump and that should be running max speed ALL THE TIME. It should never be less than 100%...and that's probably around 3000rpm. You have to make an adjustment (usually in BIOS) to change the fan/pump speed to voltage controlled, 12V at all temperatures.
When adjusted like that it will vary in speed a little bit since the +12V line from the PSU also varies a little bit depending on how you're using the system. So all you're looking for is that it run 'around' 3000, with variances expected and allowed. Some monitoring utilities report the +12V line voltage if you want to see how it varies.
But if what you mean is the radiator fans then 3000rpm is way to fast, probably screaming loud. I'd make them really slow most of the time, around 1000-1200rpm max.