I have a plank routine in my exercise days - about every other day. Basically, it's ten minutes where I do either 1 minute plank/1 minute rest or 30 sec plank/30 sec rest until the 10 minutes are up. I do the planks on my elbows.
I don't know if that seems to easy/hard for you, but you could vary the durations as needed. (I recently moved up 35 sec plank/25 rest.)
However you time your sets I suggest you pay attention to how you do the work. I don't just hold the position, I squeeze my ads tightly as much as I can. A bit of glute and quads tighten too. I think this is what trainers call posterior pelvic tilt. The point is to maximize the work of the muscles not simply hold a position. You want to avoid a limp, saggy plank. 'Walking' on your hands to get in and out of the plank position is another way to keep up the work on your core when doing planks.