Most cb-1's get about 100 miles or 160km from full to reserve. If you are riding it "like you stole it" it can go as low as 60 miles 100 km.
The fuel petcock is lever sideways off, lever down run and lever up reserve. When the bike seems to be losing power and starts slowing, switch to reserve. After riding it for awhile you will become familiar with how far your riding style will allow you on a tank.
Where can I find this fuel petcock?