This may depend on the amount of traffic you receive. For sites that do not receive a lot of daily activity, a longer length of time may be necessary to get a better idea of how the site is performing. If you have a very busy site, one week may be all the time you need to gauge how effective a new design or ad campaign has been.
If you are running an ad campaign for a specific amount of time, it’s a good idea to set the test to run before and a little bit after the campaign to get a better idea of overall performance.
It is really up to you as to how long your length of time should last. However, for the best results, we recommend at least one week.