Speaking from a "Value" standpoint: In formation practice, 1 minute yields the highest return per minute. 2,3,4,5,6,7, 8, and 9 minutes appear to be fractional increases to the 1 minute. But 10 minutes seems to (on average) double the results of 1 minute. HOWEVER, I have no amount that will double the value of 10 minutes. I have tried all the way up to 30 minutes in one slot.
There are several other ways to look at this. Above I explained Value but there is also absolute. You can go with 25 minutes in your favorite offense, 10 in your 2 favorite defenses and never go lower than 10 in special teams. 25 minutes (with all conditions the same-which they never are) will yield more then 20, and 20 more then 15 etc.etc.
And then there is the one minute in all formations and load up the rest in individual practices(perhaps best approach for d3).
D1 is probably better off with 10 in your 2 best offense, 10 in a base defense(43,34, or perhaps 44), 10 in a nickel, and the 1 minute in the dime and 52. Dime and 52 are used alot less then other defesnses.