These vary according to the race. The Saturday race is easy; everyone starts at the same time. The Sunday race is busier, and so to avoid a dangerous pile-up between the largest and smallest boats the starts are phased. The exact times are on the club noticeboard with the class flags.
The Wednesday race also has a phased start to ease congestion; again the current details are on the board.
For a pursuit race, first find your start time on the list in the clubhouse. If you're an RS400 sailor, go and have a cup of tea (why didn't you stay in bed a bit longer?) because your start is many minutes after the Topper, who should get on the water quickly. The race crew sound the horn at one minute intervals. They hold the start number up for 20 seconds before the horn until 10 seconds after. You start on the horn when your number is showing.