IANA expert on long-haul but I can throw out a few tidbits of US & my employer’s practice. Last I knew when I left SDMB for my sabbatical, we had no FAs on the board though IIRC there were a couple of former FAs who had done the job for a couple of years, not as careerists.
As a general bit of FAA regulation you need 1 FA per 50 pax or fraction thereof. As an example, our 777-300s that run ultra long-haul have 304 pax seats which imply 7 FAs. But another tidbit of the regs actually requires 10 due to the typical long-ish duration and the practical need to take breaks. 787s, 777-200s, & A330s are just smaller enough to get by with 8 FAs for long haul.
On longer long-haul an extra FA is required if the duty day exceeds 14 hours, another if > 16 hours, and another if >18 hours. For regulatory purposes “duty day” starts when you clock in for work and ends when you park the jet on the last flight before a hotel stay. Typically clock-in is 90 minutes before the jet is scheduled to depart. Which also means you’ve been on premises about a half hour before clock-in because long haul flight don’t leave from small airports with close-in parking, nor with short walks from curbside to the gate. Oh well at least you’re not getting paid for any of that time at work pre- or post-clockin. Crazy business.
Long haul rarely flies more than one flight between hotel stay, but it occasionally happens. For simplicity let’s assume a single flight.
So the max case of 13 FAs on an 18h+ duty day would be working a 16.5h+ flight.
In general there’s an FAA requirement for somebody to do a cabin walkthrough every IIRC 20 minutes to look for stuff out of the ordinary: anyone who appears unconscious or dead, water where it doesn’t belong, etc. A lot of the trash runs up & down the aisle are disguised FAA cabin checks. If the seat belt sign is on for a long time (>1h ??) there’s also a requirement to walk through to check compliance periodically*.
From a regulatory POV after that the FAs are pretty well on their own. Their time is spent however the employer wants them to do their customer service jobs, not their FAA-mandated safety job.
When the customer service work is done then practically speaking they’re on their own. I beleive our carrier forbids sitting in a passenger seat and forbids magazine reading unless in a break area out of sight of customers.
As a practical matter a lot of yakking takes place and nowadays a lot of fiddling with their phone / tablet connected to the aircraft wifi. Plenty of paperbacks are brought to defeat the no magazines rule. Which rule probably dates unchanged from the days when we carried magazines to hand out and they didn’t want the cabin staff bogarting them all.
There is an organized shift system where each of the FA’s have an opportunity for a nap. I dont have the specifics, which would vary by aircraft type, carrier, and length of flight.
On shorter long-haul flights on smaller aircraft, e.g. 767, their “rest area”, like the pilots’ rest area, could be nothing more than a business class seat hidden behind a curtain. But right next to a galley so fully exposed to bright lights & noise. Each person takes their e.g. 2 hour turn “resting” there.
On longer flights on bigger aircraft, e.g. A350, 777, A380, a dedicated “bunkroom” is installed someplace backstage with a few beds akin to the sleeper cars on a railroad train. Typically it’s upstairs in the crown of the fuselage.
Overall, the FA’s work life is much more lightly regulated than a pilot’s is. So it falls much more to employer-specific procedures about how to do their job and how to wile away the excess time after that. And to a much larger degree than the pilots, what really happens in the cabin on a day-to-day basis is a result of their own informal but very well-understood workplace culture, not the line-by-line minutiae of their employer’s procedures manual.
* At my carrier at least they don't provide the pilots with much if any info on the details of the FA's regulations or procedures or duties beyond emergency ops. Hence all the ??s and IIRCs in my comments here.