The practice time is just set up to split 180 minutes total. It does not actually represent all of your players practicing every formation.
We are aware that it seems strange that you have a total of 180 minutes and can distribute that among all of your position players, but when it comes to Formation practice, it seems to assign the minutes across all the players - offense, defense, and special teams. This is really just a balancing mechanic and not really meant to represent how your team practices those formation.
So if you did have 0 minutes for your offensive formations and 30 minutes in your defensive formations, you might be wondering what your offensive players are doing during those extra 30 minutes. We would ideally like to improve this so it made more sense, but currently all of your practice times comes out of the same pool whether your Formation practice applies to every player or not.