They start to show once you've lost fat in your stomach. But if you want to speed the process up here:
1000 abs every day. Sit ups crunches anything.
Sprints. 10, then ten v-ups,10, then ten pushups, 10, then ten burpees, 10. Rest
Insanity, Insanity Asylum, P90x
V-ups, sets of 100.
Push ups, and hanging v-ups on a bar.
Abs become visual at about 10% body fat
depends on a lot of things, Body types (Endo, Ecto, Meso etc.) your BMI, what you eat daily, how much abs you had to start with (if you previously did ab workouts) etc.
Once you lose enough stomach fat.