The hours worked will vary depending on grade, specialism and hospital. There are standard working weeks with additional "on calls" as follows:
A standard working week for a doctor is 9am to 5pm, Monday - Friday = 40 hours.
In addition to standard 40 hours doctors usually work on calls. On-Call's are evenings and weekends. When referring to an evening then it will be from 5pm to 9am, making an evening On-Call 16 hours. A weekend 'On-Call' will begin from 9am Saturday to 9am Monday = 48 hours.
A Doctor working as a locum for one month on a 1 in 5 on call will therefore work a minimum of 56 hours each week with a possibility of one weekend (worth 48 hours).
The On-Call ratio can be anything i.e. 1:3, 1:4, 1:5, etc
There are also shift systems that are more commonly used in A&E (Accident and Emergency). As the department is open 24x7 there are no standard hours of work and a shift system is implemented, the hours can range from 40 - 70+ hours each week.
In some specialisms hospital work & pay is organised by sessions. These are usually Anesthetics Doctors as they are booked by theatre time; they do a morning and an afternoon therefore 2 sessions each day.
Most locum doctors want to work as many hours as possible as you are paid for every hour you are on call. If you are only working for a couple of weeks this might be practical, but be careful not to overbook yourself. The pace of work in some UK hospitals is quite frantic (some Accident and Emergency Units see over 500 patients per day). Global Medics will always try to find you as many hours as possible in accordance with your instructions, even if that means having to do additional shifts at other hospitals.