The role lasts for 4 years, at which time there will be a review. The types of placements will vary. For example, one might be a 2 day diagnostic exercise, while another might require a 3 month, full time support role. Time may be taken as a block of consecutive days or spread over a longer period.
There is no minimum or maximum time commitment. You and your school will need to think carefully about capacity and negotiate your availability together.