Mediation is a voluntary process so no one can compel participation.
Sometimes the parties will determine ahead of time how much time they need to spend at the mediation. For example, in Hong Kong, the Mediation Notice (Practice Direction 31) will include a specified minimum participation level e.g. how long will the mediation be?
In addition, the agreement to mediate will usually include a clause requiring co-operation and good faith from both sides.
Bear in mind, this is the minimum. If you want to get the most out of the mediation session then approach it with an open mind and heart.