How fast?
A helicopter cannot realistically fly from Europe to the U.S. at its own top speed in one normal hop; the limiting factor is range, not just speed. In practice, a helicopter crossing the Atlantic would usually take many hours and multiple refueling stops, and the total journey is often around a day or more depending on the route and aircraft.
Typical speed
Most helicopters cruise around 150 mph, or about 240 km/h. Faster experimental or compound designs can go much quicker, with recorded speeds above 290 mph, but those are not typical passenger helicopters and do not solve the fuel- range problem for an ocean crossing.
What the crossing looks like
The first civilian transatlantic helicopter ferry flight covered about 3,720 nautical miles in 37 hours and 11 minutes, at an average ground speed of 100.2 knots. That gives a good real-world sense of the trip: even when a helicopter can make the journey, it is slow compared with fixed-wing aircraft and requires careful planning and support.
So what is the answer?
If you mean “how fast can it go” in pure speed terms, some helicopters can reach roughly 255 knots in special test conditions, but ordinary helicopters are much slower. If you mean “how long would it take to get from Europe to the U.S.,” the honest answer is that a helicopter can do it only with stops, and the total time is measured in many hours, not a quick nonstop flight.