How to calculate frequency of this Op Amp square wave oscillator?
You should easily be able to calculate the min and max voltage at the positive terminal of the (assumed ideal) op amp based on the resistor values R1-3 and the supply voltage... (Unknown, but will fall out of the math later).

The capacitor will charge and discharge with a time constant of C x R4 ..
It will charge to the high voltage at the op amp's positive terminal and discharge to the low voltage. Voltage at the capacitor will be equal to the supply voltage x 1 - e raised to the power of negative time divided by the time constant above... if I recall Electrical engineering 101 ... So basically solve the equation twice, once for the high voltage and once for the low voltage. The result will be the high output pulse and low output pulse widths. Add for the period of oscillation... Now add a buck and you can have a Tall Starbucks... or buy me one.