I was doing 2 sessions a week with a PT, but I teach Krav and do BJJ and Muay Thai on top of that so that was all I needed. You could maybe do 3 sessions a week but I wouldnt do more than that.

And it depends what you mean by core work. Most of my workouts were based around traditional strength training stuff. Compound lifts like deadlifts, squats, cleans, overhead presses etc. The core works to stabilise the structure and form of movements like this, particularly the deep abdominal muscles and the core muscles in your posterior chain, and in that sense they're a foundation, but they were mostly worked through paying very close attention to form in these compound lifts. I didn't a few isolated core exercises to supplement this, but there wasn't all too much done on the core.

It depends what kind of thing you're going for though. I was mainly training for functional strength for fighting/sport etc., but if you just want to get ripped your program would probably be a fairly different from mine.