I haven't timed mine, but that sounds about right. For instance, when you open your humi to retrieve a stick or just admire your stock, the RH will drop a little so the fan will initially run a short period until the RH gets back to the set point. Once there, the the fan only comes on for short bursts to circulate the air. You have a problem with the seal on your humi (or you have a new humi that hasn't been seasoned properly) when the fan runs constantly and doesn't get the RH to the set level.
Sounds like your CO is running fine.
Hope this helps.
-Dion