This depends on a number of factors, including the number of frames per second (18 or 24), whether it is normal8 or super8, and so on.
Below is the table for the most commonly used format (18 frames per second and super8):

Reel diameter Minutes
7 centimeter 3,5 minutes
13 centimeter 14 minutes
18 centimeter 28 minutes
21 centimeter 40 minutes
25 centimeter 53 minutes

Click here for the complete table with the duration of films (Dutch).