So long as the parties are making progress, a mediator should press forward. Often, this makes for long days. I recently participated in a mediation that concluded at 10:30 p.m. I think my personal record is 1:30 a.m. Both cases settled.
Sometimes it makes sense to continue the mediation to another day. However, that is not always possible. Representatives may have flown in from out of town and may not be able to stay another day. Continuing the mediation to another day may also risk losing any momentum that has developed toward settlement, but that depends on the particular circumstances.
In any event, iron in the pants is a good quality for a mediator to have.