This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
There is no such limit as far as I know. I tend to look at it the other way. As long as the job is interesting and pays well people will stick around. I know a person that left after 2 months. His job was to run a report and send an email, when in fact he wanted to be a developer. His case cannot be considered 'job hopping'. So there is no general answer to your question. Was the question raised to address a specific problem you have ?