It depends a lot on your age, as well as other factors. @bobbinhood and @syz and @janbb are all correct. You must eat a proper diet – one that you can actually stick to – that becomes a healthy lifestyle choice and is no longer a diet.
When I was in my 20s, if I got off track, all it took was 3 weeks of exercise to get back in shape. Now in my mid 30s, it has taken years of hard work and dedication to get my body back to my pre-pregnancy shape.
Your best bet is to combine strength training with the Zumba in order to build muscle as well as have a change of your eating habits. Sadly, this means no “cheat days” with fast food, sodas, ice cream and the like. One really great tip I learned was to switch around the “dinner meal”, meaning to eat the potatoes, rice, and other starch carbs at lunch and eat TONS more vegetables at night. And please don’t starve your body or buy into fad diets. They only give you short term results that may actually cause you more weight gain in the long run.
If you are only doing exercise twice a week, you will not see the results that you desire. You are going to need to push yourself and confuse your body to get it to change the way you want it to. I recommend 3 x a week strength training and 2 days of cardio (includes Zumba)
Yoga and Pilates are a GREAT way to shock your body… there is a lot of core work involved in these programs and you will need to strengthen your core overall to see any progress as well.
Sorry to give you the bad news, but there is no quick fix, no pill that can help you get there any faster. Adapting a new lifestyle of eating and consistent exercise will get you there. Also, maybe get a subscription to Oxygen Magazine for Women. It is simply the best publication to help.