A good question came in video a Google webmaster hangout, in the video at the 21:30 mark the SEO asked if Google has a recommendation around how long A/B tests should run as to not cause issues for Google or the web site in search.

The SEO said that some A/B tests require 6+ months of running to get a large enough sample size in order to validate any testing.

John Mueller from Google said Google has no specific time period for how long you can run A/B tests. The more important thing is to make sure the content on the A/B testing pages are somewhat equivalent, if they are not, you probably want to run those tests quickly and not extend them too long.

Here is the transcription:

Google does need to write up their A/B testing guidelines for search already.

Forum discussion at YouTube.