There are several variables, but on average, the light will come on around the 4 to 5 PSI range. Chevy engines require approx 1 PSI for every 100 RPM, so if you're at a 700 RPM idle you "should" have an absolute minimum of 7 PSI oil pressure. Anything lower than that and the lamp will start to flicker. If it's really low (around 4 to 5 PSI), the lamp will stay solid.
Ed