There are two conditions where a headline wins and an experiment ends -- hard convergence and soft convergence.
Hard convergence is the usual method where there is 95% confidence that one headline is more successful at attracting clicks and attention.
Soft convergence handles the case of two roughly equal headlines. If 20 minutes have passed and we're 95% confident that no headline is better by a margin of 25%, the leading headline will win. We have this alternate method in place to deliver test results more quickly, choosing the winner by a small margin rather than letting the test drag on.