I understand that F18 races are windward/leewards. How long are the legs?

Depends on wind velocity. A race usually consists of two windward and two downwind legs. The marks are set at a distance relative to the wind speed. A race usually takes an hour to complete.

As anon said, most RCs try to have one hour races.
The wind velocity affects the mark setting AND the race course selected(once, twice, three times around).
Sometimes the venue limits your choices.
It is easy to run out of lake on a windy day.
It may be a shallow bay with little deep water.
Or there may be a busy shipping channel to stay clear of.