The great thing about long exposures on film comes from the reciprocity effect. It means that there is almost no difference between a 1h exposure and a 3h exposure.
On film I always rather overexpose, as the negatives are rather 'printable' than when underexposed.
The conclusion: If you think about exposing some minutes, go for a full hour. If you think about an hour, go for 5....
Here's an example of a 10h exposure. It was a dark night. Exposed on some Fuji film and cross-processed.
Hope that makes sense!
Wow! Love your comment and the shot! Reciprocity is what was pushing me off film until I tried it for night scenes. Now it’s what brings me the fun in shooting it. For most films I find reciprocity scales but I couldn’t do so for ultra max so in that shot I went with exactly the rule you state. Thanks for the good advice :) and keep sharing knowledge