honestly since you are overwieght any excersize in general is good! because your body isn't used to excersizing just taking the initiative and doing it even once a week is a good start. its good that you started to take an interest in your health. but you don't need somebody to tell you how much to do in a week do what is comfortable, maybe set a goal and ride your bike around the block once everyday and than after awhile go longer distances. if you like riding your bike you'll end up riding it more and more all the time, at first youprobably wont be able to go super long distances but if you keep riding it your leg muscles will grow and you'll be able to bike easlier and go farther and the bigger your muscles the faster your metabolism so it will help you lose weight too! i ride my bike whenever the weather is warm but when i started out i couldn't go very far, especially if it involved going uplots of hills cause thouse can kill you! but if you stick to it your thigh muscles will grow pretty fast, and you'll see improvements almost right away trust me i'm average weight and i noticed dramatic changes my thigh muscles got so tight and firm!
