Everyone's body is different, so there's not a one-size-fits-all answer to how long it will take to see changes in your body when you start working out. It also depends largely on the kind of workouts you're doing, and what your lifestyle is like. At a physiological and hormonal level, changes start to occur in the body instantly after a workout, but most people will start actually seeing results within the first month or two of making regular workouts part of their weekly routine. These results will start small but will improve and become more noticeable as time goes on.
Get Your Expectations Right
The rate of change your body will experience is largely determined by the condition your body is in when you begin training. If you've never exercised before, you will start seeing noticeable results very quickly. Even if your workouts are basic, you'll experience obvious increases in strength, fitness and endurance after the first couple weeks of training. If you are at an intermediate level, or have trained on and off for some time, you'll more likely see results after the first month or two, providing you are training at sufficient intensity. For an advanced exerciser, such as an athlete or person who has been training consistently for years, noticeable increases in strength or fitness will take concerted effort over a couple of months, as the body is already highly conditioned.
Working out once every couple of weeks is not going to give you much in the way of noticeable changes. You might feel good once you've done it, but you won't lose any weight, gain any muscle definition or get healthier. Including three to five workouts a week can help you see changes in your weight, health and your body's appearance within the first month or two of starting training. Building this kind of habit takes time, especially if you've never done much exercise before. If you stick with it, and learn to view exercise as an essential part of your day, like brushing your teeth or going to work, it will become easier to stick with.
Go For Intensity
While the duration of your workout is important, intensity trumps length every time. You will get better results from going all out for 15 minutes than you will from strolling along for an hour. In order for your workouts to really have an impact, they need to be challenging. Your body is unlikely to respond noticeably to gentle exercise -- it needs to be tough. Not so tough that you're in pain or unable to move by the end of it, but enough that you are worn out and don't feel capable of doing any more. Ensuring your workouts are demanding will help you to see results more quickly than if you are just doing easy, but long, workouts.
Always Aim Higher
In order to make sure your body continues to change from your workouts, it's important to follow the principle of progressive overload -- increasing the number of repetitions of an exercise or increasing the difficulty of your workout each time you train. Progressive overload is what causes your body to adapt and improve, so if you want to see consistent results from your workouts, be sure to make use of this. However, it's also important to take sufficient amounts of rest. It's very tempting when you start seeing results to train more often, but your body needs regular rest if it's going to keep changing. One or two days a week without any serious physical activity will help keep your body injury-free and rested.