This is largely dependant on experience and a keen ear for what is happening to the noise from the motor. Feed rate varies with the composition of the material being cut, its hardness and the type of cutter being used.
For instance, a single flute cutter will clear faster than a two flute one so you can feed it more quickly, although the resulting finish may not be as good.
If you feed too tentatively, heat is generated in the cutter and the friction causes the timber to burn and the cutter edges will overheat and suffer damage.
On the other hand, if you try and go too quickly the waste material will not clear fast enough, the router will slow down and again the finish will be poor.