it depends at that age I was really fat but and still kind am and I can ride about 50 miles if you let me, and still keep going some times, it all depends on your endurance levels. I like to do competitive bike riding and its hard for me to loose weight, so I stay fat like even tho I ride my bike so much.
if you get a lot of exercise or are at least quite active, 5-7 miles
if your kinda active and do things some times like once or twice a week 2-4
if you never do anything and are a couch potato and stairs give you a problem maybe a mile