We have a limit laid down, normal shift not to be more than 12hrs, the only exception to this is in an AOG situation where we can work upto 16hrs max. Minimum rest is 11hrs between shifts.
The european working time directive is applicable if you state that you do not want to be exempt it.
A third party airline we used to handle had strict limits on what we could work and they could take us to court if we worked their a/c and had exceeded the hours (I think it was 12 or 14)