A session is generally considered to be around four hours. Often a locum may be working two sessions in two different practices in one day so it is important to realise that session finish times are very important to a locum.
Generally, you can either reach an agreement on a sessional fee either on a time-basis (an agreed fee for specified number of hours) or based on a defined workload eg: number of patients to be seen.
The time-based approach still needs scrutiny of the workload as it is in no-one’s interest for an inappropriately high workload to be squeezed in. This leads to long waits for patients and a very unhappy locum who will be thinking of the impact on their next practice or who’s going to collect the kids from school and making a note not to work at your practice again. Although the locum may well charge extra for overrunning due to a high workload (assuming they actually have the capacity - they may well be expected elsewhere as duty GP), it’s still not acceptable practise since this can lead to lapses in patient care.
The workload-based approach has the advantage to the locum and the practice that each party clearly understands the boundaries. A practice would not normally be charged for overrunning, unless because of unusually high patient demand (eg multiple acutely unwell patients requiring the lengthy process of hospital admission) or because of delay caused by the practice e.g. poor preparation by the practice such as inadequately equipped consultation room or locked out of the computer system.