It is great to have people join for the short term, but it is not enough. It takes time to get to know the language and the culture well. In the future, local people will carry the Gospel to every corner of their community, but for the moment there is still a need for people to come from outside and to get past the language and cultural barriers to be Jesus to them. How long is long term? It is not about numbers. It is about heart attitude and commitment to God, being there for as long as it takes. It might be five years. It might be fifteen. It might be fifty. What matters is cooperating with God to get the job done.