Time constant for RC common emitter circruit
Follow the current flow.
When Q1 is off, C1 charges from the current flowing in the series circuit composed of R2, C1 and R1. Since R1 &amp; R2 are in series you can simply add them together in this case.
When Q1 is on you have a different case. Now C1 is discharging through the series circuit of Q1, C1 and R1.
So in short - your charging time constant would be related to C1*(R1+R2) while your discharging time constant would be related to C1*R1 (assuming Q1 behaves as a perfect switch).