The duration of the pulse is can be calculated by V = L*di/dt to find the di/dt which is the rate the current decreases at. V is the peak voltage of the spike induced by the inductance (probably what you clamped it down to with your flyback diodes). You need to use all the inductance, including motors and wires and everything else the current flows through along the current path. Whether the current actually reaches zero before your next switching time is another matter
You now have current, voltage, time, and frequency. Take a good *guess* what the waveform will look like to find the power. It's not an exact science. It might be a exponential decay, it might be a linaer one. Take a conservative guess.
Great question. I was thinking about it 2 weeks ago. And after staring at the circuit and running through it, this is what I have concluded:
When you load the motor terminals down to zero voltage, it brakes the motor since the motor is acting like a generator trying to drive a very heavy load. If you did this to give the flyback current a path to flow to supress the voltage spike (generated from the inductance forcing the current to continue to flow and only change gradually and increasing the voltage to do so), once the energy from the inductance of the windings was dumped, the motor woudl start to slow down due to this breaking action (the BEMF of the motor would now start acting as a generator since all the energy from the winding inductance is gone). But...stare at your circuit carefully...the flyback diode isn't driving your motor terminals to 0V...it's just clamping the motor terminal voltage- probably to the supply voltage + the diode voltage drop which is about equal to the motor's back maximum EMF (ie. maximum speed, maximum operating voltage of the motor). So once the energy from the inductance fo the windings is dumped and the voltage spike has been supressed and the motor temrinal voltage has fallen back down to be equal to the and the supply voltage (which is about equal to the BEMF), the diodes turn off and don't driive the motor terminal voltage any lower so the motor coasts. It does not slow down because the terminal voltage is the BEMF voltage which means the motor is acting like a generator driving no-load, so no braking action.
So it all has to do with CLAMPING the motor terminal voltage to it's back EMF voltage (which is about the same as the supply voltage). If you clamped it to something lower than the BEMF, then you would start to get braking action. If you clamped it to higher than the BEMF, you would get no braking action but you would get a higher voltage spike.
Flyback diodes are wired to only give the current a path to flow IF the voltage spike exceeds the motor back EMF. In this way, they supress the voltage spike to be withing operating limits of the motor and electronics, but not to slow the motor down.