I don't know about the North Sea platoforms, but in the Gulf of Mexico, they run the gamut from barely fast enough to keep up with a running 206 to so strong you'd better hold on tight when you pull the trigger. As a guess, anywhere from 5 gal/minute to 50 gal/min. It depends on the pump, and where it and the tank are located, and on the size of the hose and piping. There are no Agustas here,
AFAIK. There are all sorts of Bells, Sikorskys, and assorted Eurocopter models, though. It generally takes just a few minutes, but it depends on how much fuel the helicopter needs and how fast the pump is. I would guess the average is between 5 & 10 minutes of actual pumping, and probably closer to 5 than 10. Lots of times we only need a few gallons, so it doesn't take long. OTOH, sometimes we're almost empty, and need to top off. As a general rule, we'll take less than a quarter of a tank, and how much that is depends on the model of helicopter.
Short answer, there is no one answer.