to the op, I won't be a condescending prick like most here love to be. They all undestand what yo uwant to know and are just choosing to be smartasses. Yes it's true that there are always going to be variables. some days your into the wind, some days its at your back. that alone will make a huge difference. I live in NYC so there are rarely places where i can ride uninterrupted. Its exactly a 13 mile ride to my golf course riding thorugh manhattan and the bronx. It takes me about 52 minutes to do that ride.
there is a 6.1 mile loop in central park that is one of the few places you can ride completely uninterrupted. I can do two laps there in right around 40 minutes. so you can see there is a 12 minute variable just based on traffic. I should also add that usually when i ride in the park, I'm riding for time and I'm pushing myself. I will also add that I'm a clyde weighing well above 200lbs. It helps a ton going downhills but hurts a whole lot more going up them!