most aperture priority cameras like the F 3 make much longer long time exposures than specified. But in such low light situations the linearity of the exposure is no longer guaranteed. You have to compensate for the so called Schwarzschild-Effekt and the camera does not know the algorithm for your film. Some cameras like the OM 2n stop after 2 minutes, others do not close the shutter again and wait for the exhaustion of the battery.