As a rule of thumb, it’s recommended to wait for a minimum period of time of 14 days to wait before analyzing the results of a test. All Nosto tests have a Data Gathering... state during the first 14 days after activation. While you can peak at the results of the variations during those 14 days, it's recommended not to end a test during that data gathering period.

Nosto shares the same positioning than Google, that expresses it clearly in the following statement: 

Why two weeks?

With tests designed to be deployed to regularly-encountered parts of a digital property, that’s generally a good period of time to get a well-rounded sense of your data, including weekdays, weekends, and any other anomalies that might happen from one week to the next. However, two weeks is a minimum, and experiments can run longer. You can also make a call earlier if you think your traffic isn’t likely to change qualitatively, although we don’t recommend doing this.

More information here: https://support.google.com/optimize/answer/7405044?hl=en

If you do decide to leave a test live for a longer period of two weeks, it's recommended to respect the full weekly cycles to avoid misleading results. We recommend you the following article here covering extensively the topic.