as others have said, there are WAY too many variables to give a straight answer to your question other than to say it will likely fall somewhere between 30 minutes and 2 hours.
my commute is ~14 miles each way. i ride everyday. the route is dead flat (chicago, duh). 8 miles of it are along the lakefront path where i don't have to deal with stop lights and car traffic and such. it usually takes me anywhere from 45 minutes up to an hour depending primarily on wind (lake michigan can get stupid windy at times) and which bike i take. in winter, the time jumps up to the 55-75 minute range (studded tires, ice, slush, snow, etc. can really slow you down).
however, i've been doing this for 4 years now. when i first started out, i wasn't strong enough to ride all 30 miles of my commute everyday, so i used a folding bike in tandem with public transit to make the journey. as i became strong enough to ride the whole way, both ways, it was taking me roughly 1 hour 20 minutes each way. as i got stronger, and as i changed from a folding bike to a hybrid bike and then to a proper road bike, i chiseled away at that time to where i am today.
Last edited by Steely Dan; 08-24-11 at 11:37 AM.