I do a quick ab routine every day, 7 days a week. I do that because I don't want size; big abs look like a pot through a shirt. I want them fine-cut, and rock-hard. I alternate among several routines..... Vertical crunches; 15-20 reps x3 or 4 sets, flat roman chair situps, 25-50 reps x 1 set, added to a fundamental 150 standard slow speed crunches plus 100 side crunches each side. So that's a total of 400-450 reps daily, and never any added resistance. I don't slack, or take a vacation from it! That doesn't work. I'm at a solid 15 years at 365 days, discount maybe 5 days/year for sickness or emergencies. Guys I see who train abs on machines or do crunches with plates, get big thick abs that look good if you're going to compete in a building contest, but it just looks like a big fat belly under a shirt. I'm 5.3% bf (last time accurately measured earlier this year), and I'm fairly close to where I want to be. My license number is 6PAK ABZ, so I have to keep up the work, or become a liar....
Charles