What should be the duration of an isometric exercise, how many sets should be done?
I'm currently doing crunches in 20-30 rep range, and now I'm going to add an isometric exercise for my obliques. I'm planning to switch between crunch, side plank / side crunch, front plank every month or two. Is it ok? Or I should do only regular exercises for abs?
Isometric exercises are great for core strength, you should definately add them to your program. Also consider that there are other dynamic exercises than just crunches. As well there is other movement functions than just flexion (crunches), anti-extension/flexion (plank) and lateral anti-flexion (Side plank). Rotational (Or more preferred is anti-rotational) is also one, so is extension. For rotational strength there are as examples, exercises like Pallof Press, Landmines, and different single-handed plank variations.
To the duration.
There is no one answer. You can alter two things: Time and intensity/difficulty. I personally don't like long isometric holds (45s-120s). I get bored easily and I don't think time is the important factor here. I usually do sets under 30s, with a different variations. It must be hard to meet the time goal, at least on the last set. That's important for me. Lately I've been doing one-armed planks for 3x20-30s. Another one of my favourites is Plank DB row, which I do for 6-10 reps for 3 sets both sides. Total time for set is somewhere around 15-20 seconds.
But there are other ways too. Goalie coach Maria Mountain uses very short plank intervals for example. Making them reps. Example is six sets of 10 second planks with 5-15 second rest between reps. Then repeat that 2-4 times.
I would say core/ab work should be done 1-3 times a week, only one or two exercises, for 2-4 sets of each exercise. If you have two exercises, I would do them either on seperate days or just once or twice a week.