Found this really useful formula for determining a song’s total time in the Songwriting Sourcebook by Rikky Rooksby (page 170):

1. Determine the number of beats per minute (e.g. 96BPM).
2. Determine the number of measures per minute by dividing the BPM by the number of beats per measure (e.g. 96/4 = 24 measures per minute).
3. Then take the total number of bars in the song and divide it by the number of measures per minute. E.g. 104 measures/24 measures per minute = 4.5m for the total time of the song.

Thanks Rikky!