Every matter is different. It will depend upon each of your communication styles, how many issues that need to be discussed and their complexity and the level of conflict present.
Some people will resolve everything in their first joint session. Most find that they resolve everything in two or three sessions, especially if they are discussing both parenting and financial issues. Some families use mediation on an ongoing basis over many years to ensure that they are continuing to meet their children’s needs and have the best arrangements in place.