Your circuit will look something like this:
simulate this circuit – Schematic created using CircuitLab
Help understanding how to use a SG3525
And the PWM frequency will depend on \$C_2\$ and \$R_6\$ values.
\$Q_1\$ together with \$R_6 , D_2 , D_3\$ forms a constant current source.
And this current will be around \$I_S = \frac{0.66V}{R_6} = 140\mu\textrm{A} \$
And this current will charge the \$C_2\$ capacitor. And if the voltage across the capacitor reaches the NE555 threshold value (2/3 Vsup). The 555 will quickly discharge the capacitor to 1/4Vsup.
So, the equation for PWM frequency will look like this
\$\Large F \approx \frac{3 \cdot I_S}{C_2 \cdot V_+} \$
So for 5V supply, as you have in your circuit the equation becomes
\$\Large F \approx 0.6 \cdot\frac{I_S}{C_2} \$