How much of a written play has to be the same to be a copy rite infringement?

Copyright infringement occurs when a substantial portion of a work is copied without permission, which is typically assessed through the "substantial similarity" test. This means that the copied material must be recognizable and significant enough that an average person would see the two works as similar. There is no specific percentage of text that constitutes infringement; rather, it's a qualitative assessment of how much the new work borrows from the original. Factors such as the originality of the copied elements and their importance to the overall work are also considered.