At the Junior level the season usually runs about 10-12 sessions of about 1 1/2 hours each. There is some work on building a different robot each week, learning about the FIRST values, talking about the project and in the end the groups give a presentation for the parents.
There is a book provided for each year with the challenge by FIRST that is geared very much toward a school format. We do not feel it needs to be run quite so formally if the mentor is comfortable taking the topic and expanding upon it to provide further learning opportunities related to the topics.
Usually each week has 1 1/2 hours allowed so there was a total time focus of one hour on robotics time. This allowed for 15 minutes of clean up, and 15 minutes of getting everyone gathered together (always a few stragglers) and focused.