I am a Work Experience officer for a large authority and can tell you that if you are a Year 10 student (Pre 16) you can be expected to work any eight hours between 7am-7pm.
There are exceptions however, which have to be accepted and agreed by the parents. For example, some placements with the military, eg , a mounted regiment, may require the student to report to the establishment by 6.30am to help groom the horses, in which case the student would finish between 3- 4pm .
Also, If you are working in a theatre your hours could be 2pm-10pm - which gives everyone a nightmare because of possible collection problems . But basically, Work Experience is meant to be about experiencing the work of the type of placement you have chosen ,and the hours that go with it.