It would depend on film choice, and if buildings are lit or not. I don't think that there is an all around rule of thumb for night exposures because you'll run into so many different lighting situations.
Bracket your shots extensively and take notes so later you can tell what worked and what didn't.
Just be reminded of the reciprocity effect. The relationship between time and exposure breaks down after 1 second exposures. I generally use the meter reading from my camera and then bracket to the next two greater times. If the meter reads 4 seconds, I take one shot there, a second shot at 8 and a third at 12. Also be reminded that both a tripod and shutter release cable will become necesary.