“When is the right time to build a new church building?”
Church buildings have a long history, though the New Testament makes no mention of them. Christians in the first century met briefly in the temple precincts in Jerusalem, preached the gospel in synagogues throughout the world, and gathered for worship, prayer and edification in private homes and possibly in borrowed or rented spaces. (We do not know, for example, who owned the “upper room” in Acts 1 or the one in Acts 20.)
The first century church was comparatively discreet and mobile. Frequent persecution tends to make that necessary. You don’t put up a sign and start construction when people are trying to kill you.