The more you ride the more you can ride (and the faster you will get).
What you ride has a LOT to do with your time. My hot rod I do a 20 mile hilly Sunday ride in a 1:35 and i'm not really pushing it. My 4.2 mile ride to work I do in 10 minutes. One of the hills I gain 130 feet in less then half a mile.

Mountain bike with slicks thats reasonably lite I'd say 50 minutes would be a good time once you get seasoned. Road bike you might knock 5-7 minutes off that time.