I lost 100 lbs on a low carb diet, but I have gained 12 pounds in 3 weeks on the weight watchers maintenance program. How long w

That depends on what you are eating.  Low carb diets are not healthy and although you can lose a substantial amount of weight following one, it is not safe nor realistic to maintain.  Your body needs carbs in order to function as it is the body's main source of fuel.  The carbs that you should be eating are those that are complex -eliminating white breads, flours, sugars. Replace those with those of whole grains instead. 

 I would need to know whether or not you are getting exercise, and what you are eating as well as how many calories you were taking in during your 'low carb" diet vs what you are consuming now to truly answer your question.