There are no direct flights from anywhere in Europe to Hawaii, you will need to fly to San Francisco or Los Angeles and change planes there for the shortest journey time. The flight to the West Coast is around 11 hours and 5 and a half to Hawaii, add a couple of hours going through immigration and customs at the first airport and you are looking at around 19 hours. The first I went I went on Air New Zealand via Los Angeles without a stopover, I was totally exhausted on arrival, the second time I stopped in St Louis for the night and found the journey a lot easier. You also need to know the time differencd is 10 hours so jet lag, particularly on the way back, can be a problem.
Loved it the first time, went back 10 years later and was less impressed. Don't spend too long in Honolulu, its just like any US city but with sun and beaches, by far the nicest island was Kauai which was relatively unspolit, hated Maui, loved the Big Island with the volcanoes.