The period of oscillation is about 1.4*R*C. R is the 160 ohm and C is 68uF. Your period is about 57Hz for this circuit. If you wanted it to be closer to 50Hz, increase R to about 180 ohms or so.
There is a problem with your circuit. The reverse breakdown voltage of the Base/Emitter junction of the transistors will be about 5 volt. You need diodes (pointing downwards) in series with the emitters to prevent this.
I can't explain how the formula is derived in this forum since my computer skills don't extend to attaching diagrams etc, so if you would like to know, send me an email and I'll send you a diagram plus the maths. It is relatively easy.
The total off time of the circuit (the sum of the off times for each q) is 0.693*R1*C1 + 0.693*R2*C2. In this case, your R1=R2, C1=C2, so it is about 1.4*R1*C1.
The charging time of the
The 0.693 is the amount of time to get 1/2 full charge given by ln(1/2)*R*C.