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A good answer would depend on more factors. You could successfully do core training one day or 7 days a week. It depends on how you go about doing it and what exactly you hope to achieve. Any more specific answers you get to this exact set of info will be limited at best one way to do it

it’s pretty much impossible to over work your core, just for the reason that it’s in constant use everyday all day. but with you being in the mma game and are most likely having some one help you or just hitting yourself with a medicine ball or something in that matter to build up the tolerance of taking punches and kicks i would limit it to probably three days a week with one really tough session of body blows. hope that helped… good luck.

4 days a week.

at least 3 times a week. Core should be the main interest of your strength training. The core is the most important muscles for grappling, and striking.

Twice.

Depends on what type of core training your talking about. If your talking sit-up’s and crunches you can do that everyday if you want. If your talking Advance training like kettle/medicine ball, TRX, or cross-fit core training it’s recommended you should do it only twice a week.