When should a church plant no longer be considered a 'plant'?
maybe churches are at their
maybe churches are at their best when they see themselves as church plants into the comunity and culture where they find themselves, focused on connecting into this "soil" and saving the lost - ie producing "fruit". a church plant is great because it is engaged and does not exist just for its members. so why lose that because we reach a certain size or age?
When should a church plant no longer be considered a CP?
I think that trying to find some 'arbitrary' factor [size, date, length of existence etc.] to answer this question may be unhelpful. I agree with some commentators that there is a need to remain connected to the wider 'body of Christ' [although this will not always be a 'mother church'] but am not convinced that this requires the continued use of the term 'plant'. I am also not convinced that what develops must be fully indigenous and self-sufficient [although this is ideal] for it to cease to be called a plant. How, if at all, is it possible for 'outsiders' [read church planters/teams] to become 'insiders' within the community they are seeking to establish / develop a church in? Is this about time spent in the community, experiences shared with the community, a commitment to a specific period of time [e.g.lifelong] etc.? Maybe a more helpful approach would be when those involved [which should include 'historically' local members and 'newer' members of the community] decide that what they are doing [growing and learning together in faith; sharing fellowship, compassion and welcome with others; expressing the Kingdom of God through 'worship' and the sacraments etc...] constitutes 'church' then it is time to stop calling oneself a 'plant'. The process of finding a name is also often a significant one, in the process of moving from a 'plant' mentality, to a 'church' mentality, which should not [hopefully] mean a maintenance mentality.
A church is ALWAYS a plant
A church plant should always be called a "plant'. There is a strong tendency in church plants to "arrive" once X or Y or Z occurs--then the vigor and excitement slowly ebbs and "normal" sets in. With "normal" then comes stagnancy at best and decay at worst.
In my view, every church is a "plant", meaning an organism that must grow constantly and keep that holy expectation that God wants us to redeem the city with the application of the Gospel of Christ in every corner of culture.
when is a church plant no longer a plant
I think a church plant is no longer a plant when it plants another church itself. Sort of like becoming a parent.
trick question
I agree with the other comment: once a church plant always a church plant. When is a daughter no longer a daughter to her father? The question is not whether it stops being a plant, but whether it exercised executive authority from the beginning, or whether at some point this will be established. Personally, I believe a church plant should always start with executive authority from within the plant, and therefore be "autonomous" in that aspect, but that it should be connected to the Body of Christ relationally, oversight etc etc.
churchplant
when it starts to minister to the local comunity it becomes a part of that comunity and not a part of some outside organisation.
other
It should never cease to be seen as a church plant. The title ( although maybe infrequently used passed a certain size of people/ length of time) should remain to maintain the essential link, oversight, vision support and accountability of the planting church... otherwise a planting church of any size, shape, age is at serious risk of becoming isolated and attacked.
My church is over 400 people in scotland and we are still a "church plant"

Church plant
OK - I agree with the comments about it always being a "church plant" - for me, particularly, because a faith community cannot thrive without a constant sense of re-invention around its externals.