Brushless DC Motor relationship with speed and duty cycle
Steady state motor speed is proportional to the voltage applied. When the PWM frequency is high, due to the inductance of the motor, the 'effective' motor voltage is controlled directly by the duty cycle (% of the supply voltage).
To find the speed voltage relationship, measure the steady state speed in rpm and divide it by the supply voltage. The constant is \$ K_{v} \$, which is almost always provided in units of \$ \frac{rpm}{V} \$. This is a very straighforward constant to measure, so most of the time even sellers of low quality/after market motors will provide it accurately.
An interesting fact that most people overlook: \$K_{\tau} = \frac{60}{2{\pi}K_{v(RPM)}} = \frac{1}{K_{v(SI)}} \$, where \$K_{\tau}\$ is the torque constant in Nm/A. That means if you know the motor's steady state speed at a constant supply voltage, and you know the maximum rated motor current, you know the maximum torque the motor can put out.