As far as I remember - and it was 2 - 3 years, it is 500 km between Okongwati where you get petrol quite reliably in the fuel depot (not petrol station) and Sesfontein, going down VZP, up and down Marienfluss, Orupembe, riverbeds down to Purros and riverbeds (Hoarusib and Hoanib if I remember the names correctly) to Sesfontein. I.e. the whole jiucy loop people came for. That is of course if there aren't any unexpected detours.
Nowadays you might be (not guaranteed supply!) able to get petrol from the drums in Otjitanda (where the Marble campsite is after the Red Drum pass), and Purros if you ask around. I would definitely check there once in as it is always good to fill up and have some reserve.