The first step in figuring out both questions is figuring out what weight you want to be at. From there you can figure out the difference between your current weight and your ideal weight. For example, say you want to get to 144 lbs. 154 lbs - 144 lbs = 10 lbs. Because it takes 3500 calories to burn 1 lb of fat , you can calculate it will take a 35,000 calorie deficit to lose 10 lbs. If you subtract 250 calories a day, it will take you around 4.6 months to reach your goal. It's important to be realistic though, and acknowledge there will be some days you eat over your caloric budget. There will also be some days where you eat under your caloric budget. Keeping both these things in your head, its important to gauge your progress on a weekly basis. There are a few ways you can do this.
Myfitness Pal is a smartphone app and website that allows you to count calories in and out via exercise. Its not something to be used as an EXACT number of calories in and out, but it will give you an idea of how the things and amounts of things you have been eating, contribute to how you now look and feel. I left a link to the homepage of Myfitness pal below.
Pay attention to how you look in the mirror. Its very easy to be biased in a situation like this, but try to honestly take inventory of how you look compared to how you have looked before. To avoid making this subjective, look for objective sings that your making weight loss progress, for example, are you more vascular? does your skin seem/look tighter?
As time goes on and you continue evaluating yourself, continue to tweak your calorie levels according to how you look, and how you want to look.
Note - Calorie counting isn't a necessary part of losing weight, but it is an extremely useful one. If you can keep track of your diet mentally then feel free to not count a single calorie. However, keeping track of food choices and quantities can without a doubt help you learn more about how your diet and body composition work together.
Myfitness pal - http://www.myfitnesspal.com/