Two-times (pun intended) a week. Wednesday and Saturday (my rest days) look like this:
Don't do core workouts on the same day as weight lifting as you simply cannot give it your max effort. You need your rest, Sunday is an off day but Wednesday and Saturday are your rest days where you keep it light but still work on conditioning.
Cardio:
5mins easy/medium warm-up
+
Core Strength Workouts:
(I don't always do it in this order as I usually focus on one exercise that I could improve or just maintain my current fitness levels)
Leg Raises (20x3)
V-sit Twist (20x3)
Hip-Thrust (20x3)
Plank (3-5 minutes)
Press-Ups (To failure)
Stomach Vacuums (10 each way)
Sit-Ups (To failure)
Pull Ups (To failure then finished with a Hang Bar to failure)
Dips (20x3)
Dragon Flag (20x3)
+
Cardio:
HIIT (2-4mins warm-up/1min steady-state/10mins HIIT 1:3 split for example 30 seconds race pace 80%-100% max then 90 seconds rest 20-30% max. As the weeks go on play with shortening the rest periods or lengthening the time spent at max pace to increase your Vo2 max)
+
Cardio:
5mins cool down
Basically, depending on your goals (generic disproportionate "body-builder" look, athletic for a certain sport or industry field like the military you should aim to train for that exercise first in your routine so that you don't deplete your glycogen stores and half arse the other exercises that would actually get you to your goals faster.