There seems to be a raging debate in the mediation community about the usefulness of joint mediation sessions vs. separate caucus sessions. Many mediators keep the parties and their attorneys in separate rooms almost from the outset. They do this to minimize animosity, and to avoid driving the parties further apart with hours of venting, accusations and counter-accusations. They do it because the parties and attorneys are often impatient to cut to the chase of negotiations, and feel no need to exchange information they already know too well. Other mediators believe there is a value in attempting to mend a broken relationship, or that the parties may need the cathartic experience of confronting each other and listening to each other before they can reach a settlement, and therefore attempt to preserve a role for the joint session.
I think there are also some efficiency arguments in favor of keeping the parties in joint session until they actually reach the stage of trading numbers back and forth. I like to find out what areas of agreement exist between the parties, which can more easily be done when everyone is in the same room. I like to have the parties listen to the other sides' attorneys' assessments of what might happen at trial. And I like to save myself the trouble and save the parties the time of having me listen to one side's story and have to repeat it to the other side in a different room. To the extent that everybody needs to understand what each side is contending, it saves a lot of time, and probably some losses in translation, to do that while everyone is in the same room, rather than by having the mediator acting as the messenger of such information.
On the other hand, I appreciate the dangers of inflaming people's passions in an unproductive way, and that it can sometimes be a waste of time to re-hash the facts of a case and the parties' respective contentions when the mediator is not there to decide the case or evaluate those contentions. Parties also need to understand how to listen to each other, and how to communicate in a persuasive way, for joint sessions to be helpful. So the answer to the question of how soon to break into caucus is, of course, that it all depends on the situation.