All airlines are different, but for the most part they structure rest breaks similarly. Typically right after takeoff is the most elaborate meal service which requires all crew to work their respective positions and takes up to 2 hours. After that, the workload on flight attendants go down and typically they will divide into 2 shifts. Depending on flight length the rest period can be anywhere from 2 to 4 hours and sometimes more. Flight attendants either get to rest first and work the second half or the opposite. When resting, crews typically will use a crew rest either overhead or under the cabin floor, although sometimes they use blocked off seats. Depending on how elaborate the second meal service is, all crew may be on duty to serve it or it may be half the crew. All crew are needed to prepare for arrival especially if a breakfast service is being done as late as possible before landing.
In general, the only time all the flight attendants are needed is for boarding, meal service and arrival. Cruise with the lights off is not demanding at all. Typically there is one flight attendant in first class, one in each business class section (if more than one like on a 747), and then 2 in economy. There may be more depending on staffing levels.
Here’s a great crew report of a long haul flight:
Air NZ Longhaul Cabin Crew Report - Day In The Life (by TG992 Jan 3 2006 in Trip Reports)
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