[Connolly said that the current plan is to start off with two lunar missions per year. That frequency was derived in part from the capabilities that were assigned to the CEV i.e. the ability to function in space for up to 6 months and the launch rate that would likely be used to support the ISS.] There is nothing magic about 6 months ... backed its way into lunar missions.  -   John Connolly     Quotes
[Connolly said that the current plan is to start off with two lunar missions per year. That frequency was derived in part from the capabilities that were assigned to the CEV i.e. the ability to function in space for up to 6 months and the launch rate that would likely be used to support the ISS.] There is nothing magic about 6 months ... backed its way into lunar missions. John Connolly