I left my Holga out overnight with the shutter open. My method to hold the shutter open was to wrap the shutter release round and round with an elastic band. Some people just cut the shutter spring, but that’s too permanent for me.
The exposure was about 6 hours give or take. I picked the camera back up before dawn, but it was already getting light, probably would have had more trails if I’d gotten up a little bit earlier.