This is up to you and there is no right answer. Formation practice will improve your players' Formation IQ ratings which will help them play better while using the associated formation. Since this time takes away from the players' positional practice time, it is a balance that you will have to find. Keep in mind that 0 minutes in the Formation practice will keep your players where they are, while 1 minute or greater will improve them. Since the improvement may be fractional, you may not see the improvement when you look at their Formation IQ ratings.