Flat abs are great, but don't forget about the rest of your body. You want to do resistace training for all your major muscle groups. Yoga and pilates are both good, so are resistance bands and even weights. Don't worry, you won't get a "body builder" look - women just don't develop that kind of muscles. And make sure you stretch well, too. It is good for your muscles after all the work, plus it gives them a long, lean look.
For abs, you want to vary your routine. Muscles get stronger as we use them and what worked 3 weeks ago isn't going to be as efficient now. Crunches are a good start. Also do cross-abdominal for the obliques (like a slow bycicle, just think ribcage to opposite hip) and hip lifts and leg dips for transverse (that's the lower abs, where most women get the "pooch") - put your hands, crossed palm down, under your hips with your legs straight up. Lift your hips, like your trying to touch the ceiling with your heel (it's a very small movement!); from the same starting position slowly lower your legs (one or both) toward the ground but stop as soon as your back starts to arch, then left back up. Another great exercise for your entire core is the plank. It's like a push-up position, on the balls of your feet, but with your forearms on the floor instead of your hands. You hold the position for as long as you can. Make sure to keep your back straight (thats what really works the abs). Also, anything requiring balance, like standing on one foot, engages those core muscles.
As far as time, I'd start right now. The results don't happen all at once. If you spent 15-20 minutes a day (I do it while I'm watching t.v. but you could just add them at the end of your exercise class - unless they're already doing abs there, then it's enough for the day) you should start getting results within a couple weeks. But how much fat is over the muscle will affect whe you start actually seeing the results. So a healthy diet (lots of fruits, veggies, whole grains, low on the fat and sugar) and regular cardio exercise are a must.