The most important element for accurate A/B test results is that you run your tests over at least two full business cycles. This will help account for any 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 will benefit from running their tests for a longer period of time. Conversely if you have a high-usage website you may not need long to generate insightful results.