How to estimate the settling time of the rotor of a stepper motor?
$$\tau=J\frac{d²\Theta}{dt²}+F\frac{d\Theta}{dt}+\tau_L$$ 
$$\tau-\tau_L=J\frac{d\Omega}{dt}+F\Omega$$ 
$$\dot{\Omega} + \frac{F}{J}\Omega=\frac{\tau_L-\tau}J$$ 

Solve the differential equation, \$\Omega\propto pulses/s\$

edit:
$$\frac{d²\Theta}{dt²}+\frac{F}{J}\cdot\frac{d\Theta}{dt}+\frac{\tau_L-\tau_nsin(\Theta_{el})}{J}=0$$ \$\Theta_{el}=\dfrac{4\Theta}{fullsetps}\$ ; you can substitute \$\Theta\$ 

Inital condition: \${\tau_L-\tau_nsin(\Theta_{el_{initial}})}=0\$ , in absence of load torque, the electic angle \$\Theta_{el_{initial}}\$ is zero, since no output torque is produced. This also means that rotor flux is alligned with stator flux. 

At time t=0, the stator winding is switched so that \$\Theta_{el}=\Theta_{el_{initial}} + \dfrac{\pi}{2}\$, the stator flux is at 90deg in relation with rotor flux (if we ommit the static load torque, that brings the rotor at initial position different than 0deg )