It's a very simple oscillator, and the frequency is:
f= 1.44 / ((R1 + 2 * R2)C)
As for how to derive that, consider the charging path, which would include both resistors in series. The output will swing high (towards the positive rail) while the voltage across C remains between (1/3)Vcc and (2/3)Vcc (set by the triple fiver's internal voltage divider). The discharge path includes just the one resistor (R2) and pulls the voltage across C back to (1/3)Vcc. The sum of those times determines the frequency.
Do keep in mind that the formula is approximate, and that the error will increase with increasing frequency, so that for high frequency operation, some component tweaking will be required