We've made numerous passages of 250-1300 miles around the Gulf and here's the schedule we use with four people:
Sailor 1 has been at the helm for two hours, sailor 2 comes fresh off of rest period and takes the helm for two hours and sailor 1 stays in the cockpit for two hours as the lookout. Two hours later sailor 3 comes off of rest and takes the helm for two hours, sailor 2 comes off the helm and is in the cockpit for two hours as the lookout, sailor 1 goes off watch. This continues for the duration of the trip.
After dark, everyone on deck wears a safety harness.
s/v Paloma, Bristol 29.9, #141
Slipped in Bahia Marina, easy access to Corpus Christi Bay and the Gulf of Mexico.