I put out enough dry food for about 12 hours. I would start by putting out about 1/2-1 cup a day. That should be gone in 24 hours, maybe a little more. I wouldn't leave it out for longer than 48 hours, less time than that is better.
As for the food, Science Diet is okay but definitely not good enough to be worth the cost, IMO. If you only have access to PETsMART or Petco, Royal Canin or Nutro would be good foods to use instead. Even better are foods you can get at pet specialty stores, such as Chicken Soup, Felidae, Pet Guard, Innova, California Natural, Solid Gold, etc. Read the ingredients on any food you choose. Look for one that has no meat by-products as a priority. Corn gluten is another "bad" ingredient. Science Diet uses it as a protein source because it is cheaper than meat.