After X days you will be able to detect changes greater than X%
Note This does not guerantuee that your test will be done nor that it will be statistical significant. Even if your test result is statistical significant after X days it does not mean that the results are valid. If you stop to early the result might be false positive.
There are a lot of factors to take in consideration when declearing a test winner. To learn more about statistical errors read this Nature article.