Most schools assign two weeks during the school year or 1 day per week for 10 -12 weeks. However, you are usually not restricted to the assigned week and can get permission from your Work Experience Coordinator to have a placement during a normal school week. This obviously depends on how work experience is coordinated in your school, but generally once the placement you have is relevant and useful to you, you should be able to take a normal school week off to do it. Extended work experience would be a bit more difficult to get if your school only permits one day per week for 10 -12 weeks.