I saw a Polaroid 250 manual that said the maximum was 10 seconds. Easy enough to test with your camera: just go in a dark room, cover the photo cell, cock the shutter, hold down the shutter button and start counting.
It doesnt measure dark and gives every control to hand at that bulb mode and after turning it to bright light after dark, it doesnt close the shutter but only with hand control. I counted 90 slowly.
it seems as if something around 10 sec. is the longest time for reliable results. in total darkness (or cell covered) the shutter stays open for much longer indeed, but it's random and depends on the model, the battery and and the circuit version. I own quite some models and they all differ from each other. but I didn't investigate any further...