To create a statistically significant experiment, you must determine how long you need to run the experiment to include enough visitors to effectively evaluate the results of the changes to your website.
If you need help determining the minimum length of your experiment, you can use the experiment design spreadsheet provided by Adobe as a tool to help you design your experiment. This file, named VS Controlled Experiment Design.xls by default, functions not only as a worksheet but also as a record of your decisions about the experiment. For more information about this file, see Experiment Design Spreadsheet.
The experiment design spreadsheet can provide useful statistical inferences only when the metric in question is defined as a percentage of visitors that meet some criteria. That is, it is useful only when trying to test a visitor-based metric hypothesis.