Actually the EU grid is pretty clean already and there is no need to replace other renewables and nuclear with offshore wind, at least not for climate reasons. Last year it was 38.8% fossil fuel, so we are talking 19,769 offshore wind turbines.

Modern offshore wind park construction ships can install one every day. That is really the problem, you need specialized ships, with trained offshore workers and proper facilities on land.and so forth. Basicly scaling it is not easy. However last year auctions were for 20GW offshore in the North Sea, which are 1333 15MW turbines and this year is going to be more. So we are talking about a bit over a decade maybe. Obviously solar is expanded as well, as is onshore wind.