We would take on fuel whenever we could, we tried to keep the tanks topped off. It was good practice, and gave us longer legs if we ever had to shit and get. I think we made it a point to refuel if we were down to 80%. I could be wrong, but I don't think we were allowed to go below 50% unless we were at war.
How long a fuel transfer would take naturally depended on how much we needed. I think fueling usually took from 1-3 hours from start to finish. We'd usually run two fuel lines, or fuel at one station and take on supplies from the other.
Here's a pick on wikipedia from the same med cruise of us refueling from the GW.
http://upload.wikimedia.org/wikipedia/en/5/5f/DD968in1996.jpg
I found this info on fuel consumption.
"For instance, the US Navy’s Spruance-class destroyers that displaced about 8000 tons carried 1400 tons of fuel, giving them an operating range (endurance) of 6000 nautical miles at 20 knots."
http://www.journal.forces.gc.ca/vo9/no1/10-haydon-eng.asp
The Spruance class, Ticonderoga class, and Arleigh Burke class are powered by four LM2500 gas turbines. While gas turbine use more fuel, it give the ships the ability to go from "cold iron" to underway in 15 minutes. The older boiler ships would need a full day to get the steam built up. A great advantage in a Pearl Harbor or 9-11 situation.