This is one of the recurring questions about hybrid cars.
Using Torque, I collected some data about my daily commute from home to work and back.
Here is one trip, from work to home.
Engine rpm are drawn in red: when the value is 0, the engine is off.
Veicle speed is blue: where speed is greater than 0 and the engine is off, we're moving with the electric motor.
By comparison, in the trip from home to work, the engine is off 46% of the time.
On the average, in my daily trips, the engine is off 50% of the time.
There's a thread about this on the Hybrid Synergy Forum.