How long to keep pudding once it has been opened up?

The general rule of thumb is 2 hours for all cold foods that are at room temperature. In other Words, if a food that is suppose to be kept cold (refrigerator or freezer) is at room temperature it should be back in the refrigerator/freezer within 2 hours. As long as you want. When it will start tasting bad is different for all kinds of puddings. Its not going to kill you, so however much flavor degradation you want is up to you.