20 is great for newer coaches for their first year or two at least. i think 20m makes a lot of sense for d3, d2, and low to mid d1, for everyone really - its at least a reasonable option. later on when you are a master of squeezing the most from all your individual practice minutes, you probably want to increase that some. in higher d1, where you recruit players who start good and don't have to grow for 4 years, most folks run 25/25. its good to slowly work your way up. a bunch of folks will also put a couple extra minutes in their defense in particular, especially if its press (so maybe like 22m press, 20m offense, or for high d1, 25m press, 23m offense, etc).
but anyway, i would just start 20/20 and when you are in a position where you feel like, you really have some extra minutes that aren't helping that much for most of your players, then you know its time to start stealing a couple of those minutes for team practice.