First of all, for night shoots sometimes the in camera metering may not be accurate. It will be good to do some bracketing.
Since most cams max out at 30s, any longer you'll have to use bulb mode. First of all, set a wide aperture like f4, and see what's the metered shutter speed, say it is 4s. Then select the aperture you want to snap at, and calculate the number of stops to compensate. Say you want to take at f11, so f4 to f11 is 3 stops difference. From 4s, 3stop difference will be 32s. So just take with bulb mode and time for 32s.
And then, there's reciprocity failure of film too. Typically for long exposures, you may have to compensate 1/2 stop to 1 stop more, depending on the film you use. Say you need to compensate 1 stop more, so you'll have to expose for 64s.
I'm not exactly experienced in long exposures, but above is the knowledge I have so far. Any misconceptions or fallacies, please do correct me!