It all depends how you you get the FAA cert and how many hours you'll fly on it. There are no minimum flight hours for FAA private helicopter if done as an add-on rating so you can do it with as few as 20 hours on a Robinson (fewer on something else) but you'll need 45 hours to convert it to a UK PPL(H). There are places in the US where you can do the CAA conversion (or at least there were when I did it).