Also if you ever feel like it after a session, do Cardio Abs just to do it. Throwing in an extra ab workout here and there is helpful, and it's short and doesn't tire you out or anything.
never done insanity or p90x or anything but this is similar to what I do, main strength routine monday, wednesday, and friday, and I do at least some core exercises six days a week going heavier on them tuesday, thursday, and saturday. seems to recover pretty quickly and it's a great area to focus on whether your goals are aesthetic or functional