Depends on a lot of things for example your starting body weight, doing crunches is not so much so an abdominal region weightloss exercise, it is more of a exercise to thicken and strengthen your core. Diet and cardio are the things that blast off the fat, the crunches if done effectively make sure that your abs are developed when the cardio and clean eating has done its job eating away at your stomach fat. Also note that it is in your best interest to not do crunches only, as your abdominal region is made up of 3 regions which are not all equally challenged by crunches. Try hanging leg raises, for lower abs and oblique twists.
Those are not the only workouts, just general ideas of how to approach getting 6 pack abs.
Yeah, relying upon ab exercises to slim the waist line is very common. This type of approach is called spot reduction and despite being probably the most common fitness technique out there, is extremely ineffective. I"ve written several hubs on weight loss; feel free to visit my profile. I'm sure the info there will help you. Good luck.
yeah jclaffee is right, not a good approach, your diet is more important. Since your a woman (I'm assuming by the name) your body requires more water than a man, and if your eating a lot of sugars, your body is retaining more fats. Without a proper diet, you could do thousands of crunches and it really wouldn't be effective.