The practice and playing season are shown in Section 17 of each of the NCAA division manuals. See especially Figure 17.2 for D1/D2 and Fig. 17.1 for D3.
Section 17.2 in each manual has info relating specifically to baseball, and that's where you'll see the 45 day D1 limitation and different limitation in the other divisions.
Also, the NCAA defines practice, skill instruction (17.1.6.2.3), conditioning (17.1.6.2.x), and "voluntary" activities (17.02.13). We tend to lump these all together as "practice", but the rules treat them separately. I think you'll find that any D1 baseball player "practices" a lot more than 45 days in the fall.
NCAA Manuals