Welcome to the router forums.
Each piece of wood is different. Density, grain direction, moisture content (and not just in the wood either). Is the bit sharp? Are you going with or against the grain? How fast are you moving the wood through the bit? Many variables in the equation. Do as Mike and Bj suggested, use test pieces, try different speeds. In time, it will become more of a "feel" than anything else.
***Please fill in your profile, at least your location. As this will help us help you better.***
"Whether active duty, retired, national guard or reserve - is someone who, at one point in their life, wrote a blank check made payable to "The United States of America", for an amount of "up to and including my life". That is HONOR, and there are way too many people in this country who no longer understand it." -Author Unknown