Students are expected to be involved for the equivalent of at least three to four hours a week over the two years in, either a balanced range of different activities, or in a single project. It is desirable that the three CAS elements should have roughly equal status in terms of time allocation in an individual student's CAS programme.
In the two years of the IB course you have to fulfill the minimum of :

A programme in which creativity , action and service are seperate activities is just as valid as a programme in which all three are integrated into a single project. However, it is a requirement that each activity has an approximately equal proportion of time allocation to maintain the overall programme balance.
These activities do not have to be based at the school.
You cannot do the activities all by yourself, you need a supervisor (a fellow student cannot be a supervisor unless she/he is over 23 years).
The CAS program as described above is a compulsory requirement for obtaining the IB diploma and therefore the program is an exam requirement for you. Should an IB candidate fail to fulfill the CAS requirement we will have to report this and it will normally result in no diploma being awarded.