Calculating gate to source voltages in h-bridge circuit
Q3,4 are same but complementary to Q5,6.  

Timing depends on Motor L, R values with FET Ciss and collector rise times and PWM rise time for Vgs rise time.  

You also need to add a dead-time (depending on transition time) to prevent Vcc to GNd short or "shoot thru" during switch crossover interval. Thus the turn OFF time must be shorter than the turn ON time, so your circuit will fail.

read more http://www.modularcircuits.com/blog/articles/h-bridge-secrets/h-bridge_drivers/