How Do You find The Cutoff Frequency of a Low Pass Filter?
R1 || R2 = 969.89 ohms. It is less than 1% of 100K but just to be accurate; 100K - 969.89 = 99,030.11 ohms = 62.22 seconds or 0.01607 HZ as a full cycle. Basically a half minute to charge or discharge the capacitor to pi.
Note that R3 dominates the time-constant for both charge and discharge, so while R1 and R2 are important in terms of current source and sink (and the maximum charge voltage for C2), they have little effect on timing.