There is no law on the length of an individual shift.
MOST workers are entitled to a minimum interval of at least 11 hours between the end of one shift and the start of the next one BUT there is an exemption where a job requires 'round the clock' staffing. If your girlfriend works in a residential care home (rather than helping people in their homes), it's likely that this exemption will apply. However, where staff don't receive a break of 11 hours between shifts, they're normally entitled to 'compensatory rest'.
See here:
http://www.direct.gov.uk/Employment/Employees/ WorkingHoursAndTimeOff/WorkingHoursAndTimeOffA rticles/fs/en?CONTENT_ID=10029451&chk=pR1H4%2B
Chris
PS: Many years ago, I worked '36 hours on, 12 hours off' continuously for several monthsat a time. I loved it and it certainly helped my bank balance ;-)