Chains can last from 800 miles to 2000 miles depending on the weather conditions you ride in, your riding style ( if your out the saddle alot powering), and your body weight. Also how you look after them, Im 15 and a half stone and my chains tend to stretch quite fast especially through the winter. I've never had a chain break on my road bikes, but I've had them snap on my mtb a few times. I think component wear is different to each user.