Church planting has three flexible phases: outreach, discovery and planting. The amount of time will vary per context so always listen for how the Holy Spirit is leading you.
Outreach consists of looking for someone interested in Jesus. This can be with strangers, existing relationships or within a group you are getting to know.
You enter the discovery phase of church planting after you find someone open to Jesus. Focus on those you've found open to Jesus and help them take steps towards him. When you find someone interested in Jesus you then help them discover Jesus through conversation, bible study, prayer, etc...
When someone has discovered Jesus as Lord and Savior now we can help them plant a church. After they come to know Jesus you help them share Jesus with their relationships and gather a group of interested in discovering Jesus. The new disciple begins to reach their friends and family, while you model and assist how to do it. Once they begin to obey and establish a small gathering only stay for 3-4 weeks. You leaving (but still coaching) helps establish them as the leader and increases ownership. Sometimes the Spirit leads us to bring them to our home church and not plant so always be listening to what Jesus is saying.