The thing to remember with abs, is that you work them in the majority of your main lifts i.e. squats, deadlifts, bench press. These all require key contraction and engagement to get the most output on the lift.
Therefore with abs being used quite a lot, training them 1-2 times per week is fine.
If you aren't doing big lifts and abs are a target area for you, then try hitting them with different workouts and intensities 2-3 times per week.
Remember, you can't burn fat off your stomach by training abs...you're simply training the muscles underneath the fat. Balanced training and diet are the key to burning abdomen fat!
Here's an awesome ab-focused plan to try in the gym: