WCF, it would be very difficult to lay down precise rules to allow cabin crew to have a break.
Yes, it is highly desirable that you have one. No, nobody in their right minds (if they think for one moment about the possible consequences) should expect you to carry on non-stop for 10 hours.
Any CSD/Purser/No1 should have developed sufficient team management expertise to be able to sort it out. Team working should be able to accommodate this.
However, there are sufficient unknowns and possible developments that may not make it possible. ("Can someone else please give them an emergency brief? I'm on my break").
Not all airlines work the same. No two teams of aircrew work quite the same. If you have a problem within your company, I suggest you sort it out with your Cabin Services Manager. If you have no joy there, contact your Flight Safety Officer. But I don't think that you will find enough commonality here to conduct a campaign through PPRuNe.