The most important element for accurate A/B test results is that you run your tests over two full business cycles at least, to account for seasonality. For example, if you see a predictable conversion trend over a week, you should set the test to last at least two weeks. A shorter period might not include seasonality effects and end with biased results.
Though not recommended, you can always stop a test early.
Low-usage sites should run their tests longer than high-usage websites.