Pros/cons of collecting more data in an A/B Test

Apologies for a 2nd post here, but just trying to learn more about A/B testing and experimentation

What is the "harm" in collecting more data or letting your experiment run longer

For example, let's assume we determine that we can reach sample size within a week for our primary metric CTR. But we need 3 weeks to measure retention, and so we determine that we must run this experiment for at least 3 weeks

What is the harm in running …

a/b testing b testing cons data datascience example experiment experimentation learn learn more least pros retention running test testing