Which St. Lucia airport will I fly into?

If you are arriving from outside the region, almost certainly Hewanorra (UVF) at Vieux Fort. George F.L. Charles (SLU) at Castries takes regional and inter-island flights. Your ticket names the one you are booked into.

How far is Hewanorra from Rodney Bay and Castries?

Roughly an hour to an hour and a half by road, depending on traffic and where exactly you are staying. Hewanorra is at the southern end of the island and the main resort areas are at the northern end.

Should I book a transfer in advance?

It is worth doing, particularly for a late arrival or if your party has a lot of luggage. Taxis serve the airport, and your hotel can usually arrange a transfer when you book the room.

How much time should I allow for my departure?

Add the drive south to whatever check-in time your airline asks for. Travellers who plan only for check-in and forget the transfer are the ones who arrive rushed.