Exercise is quite likely the most important step that you can take to feel better, look better, live longer, and prevent chronic disease and other conditions. This is not Rocket Science. Expert after expert after expert say that at a minimum everyone should aim for at least 30 minutes of physical activity every day. Every day! Don't skimp some days and say you will make it up the next day. You have to make it a habit and there is significant research behind the recommendation to exercise at least 30 minutes every day. Of course, it is always recommended to see your medical professional if you have been sedentary before starting an exercise program.
30 minutes every day may sound like alot but it is not. Think about how you spend your day and I guarantee you can find 30 minutes a day to exercise. Remember - it is the minimum recommended amount. Also remember that it does not have to be the same type of exercise every day, there are hundreds of ways to exercise (although a good way to start is just to lace up some sneakers and start a daily walking routine), and you can even split it into two 15 minutes sessions or three 10 minute sessions.
So, if you are starting out - start with 30 minutes but recognize that if time permits and as your fitness level increases you will reap even additional short and long term health benefits by increasing the time. Increasing to 45 minutes a day and if possible, to one hour a day are shown to add additional years to your life.