I have wanted a way to know the interval I should use depending on how many hours the images are taken and the length of movie I want. I think I have a formula that works.

Hours divided by minutes times 2 equals the interval.

For instance I want a time lapse taken over 3 hours that will last 1.5 minutes when I watch it.

3 hours/1.5 minutes = 2, 2*2 = 4. The interval is 4.