I think it lasts over the session, so for example if the user closes the browser she will no longer be in the Visitor group (unless again referred by the same site).
The content served is the first one that matches a visitor group. So if you have the 20PercGroup block before the 10PercGroup block in the editor and the user matches the 20PercGroup then that content will be served. The 10PercGroup can be served only if the user does not match the 20PercGroup block.
Is there any documentation about how long visitors belong to a visitor group, and what happens when they are pass the criteria for multiple groups?
For example, my website visitor is refered to my site from a discount site and he is shown product prices with a 10% discount (he is in the 10% discount visitor group). He closes his browser and comes back to my site via a Google search 2 weeks later. Is he still in the 10% discount visitor group?
Can he be in multiple visitor groups at once? 10% discount and 20% discount for example?