Here is Multiplying Missional Communities that expands on the Leadership Development in Community series focusing on the Missional Community Leader.  (Eugene gets credit for the this one.)  He writes regarding when/how to replicate:

Expect the Spirit to work in and through Community

As our communities gather rdinary people doing ordinary things with gospel intentionality, we should both pray for and expect the Spirit to work among us.  Asking for Wisdom: As your MC reaches 18-20 you should begin to pray for wisdom, guidance, and direction in earnest.

Organic Cues: Like those who disbelieved Peter stood at the oor (Acts 12) we can often pray without faith. When the right time to plant comes we can expect the spirit to use
organic cues or natural signs.

– Major life changes (i.e. –having a baby,)
– Mercy opportunities arising
– People moving into areas of the city your MC hoped or needed to plant anyway.