How often should I lift lower body/do crunches?

during the off season?

At minimum, to really be athletic, no less than 100 reps (preferably a mixture e.g. 20x Sit Ups 20x crunches 20x Drop Series and 40x Flutter kick) of abdominal and oblique rotational/isolation exercises 3-4 times a week. Do this AFTER compound lifting if performed on the same day and stretch afterwards. The Seal, look it up, best stretch out there. And like kyleb says guys, DON’T FORGET YOUR COMPOUND LIFTS! Anything more convenient than lifting multiple muscles at the same time? No.

Sit-ups and crunches are poor exercises because they can shorten the rectus abdominus and depress the rib cage. I recommend reverse crunches:

As well as planks (both side and front) that work to stabilize your core.