The fundamental frequency is determined by the commutator switching speed (which varies with motor rpm), but the rf produced by arcing can go up to GHz frequencies.
Here's an example:-
The blue line is before adding ferrite cores to the motor leads, the green line after.
At lower radio frequencies (below 100MHz or so) small capacitors connected from each motor terminal to the case (possibly combined with small inductors in series with the leads) are usually sufficient to reduce EMI to an acceptable level. At GHz frequencies the wiring itself blocks it, but various internal parts of the motor can act as antennas. If the motor has large cooling slots or a plastic end bell these frequencies can easily radiate.
At audio frequencies EMI is mostly conducted through the wires as a result of the pulsing current. This is mitigated by twisting and/or shielding the motor wires and keeping sensitive circuits away from them, putting large capacitors across the supply rails, avoiding ground loops etc. In severe cases the motor might need to be on a separate power supply.