There's a little more than a week left to the Plank Challenge I'm doing. Recently, I've held a 4 minute plank but I'm planning to get to 5 minutes by the end of the month.
I know I already have strong abs... you just can't see the muscle through the skin. I do a lot of sit ups, crunches, side planks, mountain climbers, etc. I do think my abs are noticably better, but I know it takes time.