It depends on how reliable you think he is. If he always seems to be breaking things and not returning stuff on time I would charge him, like someone else said, $5 an hour. That's actually a really good deal for renting a bike. But if he really needs it, you can count on him, and you don't need the money, I say loan it to him for free. That way if you ever need him to do you any favors just say, "Hey remember that time I loaned you my bike for FREE?"