Determine MC34063AP Duty Cycle
Look up the AN920A/D and slva252b.pdf (Texas Instruments) for all the gory details on how this chip works.  There are several free MC34063 calculator programs available that help out with the component values.

It's not a super trivial process to do by hand.

The maximum Ton:Toff ratio is 6:1, assuming no current limits are reduce that and are controlled by the charge capacitor and the chip current sources, reference sections 1.2-1.4 of the TI note.