How much should I exercise?

I'm 26 years old, female, 5'4'' and currently weigh 155. I used to be 135 lbs which is a good weight for me. I want to get back in shape and lose about 20 lbs. I know how much to eat and what to eat to lose weight but I'm not sure how much I should exercise. Is 30 mins cardio plus some arm and leg strength exercises enough? 6 days a week? I want to lose 2 lbs a week. Going on vacation in about 2 months. I plan on eatin 1200 calories a day and lots of water. No starch and no junk.