Here I would say that, One size does not fit all. Every team deals QA differently. It so much depends on the project you are working on, either it's a small one or big one. Does it need extensive regression, User acceptance and exploratory testing or you have quite few scenarios to test.
Let me restate that in Agile, generalist are preferred on specialist. What is that? Because there is time during the project when you don't have anything to Test, so at that time you might be doing something else. Also you might be doing testing even though you are a hard-core programmer.
How do we handle it?
We have regular 2 week sprint. Testing start after a week on the task completed by developers during the week. Now tester keep adding issues to our Issue tracker and developers who are done with their sprint tasks start picking those bugs. By the end of the sprint we mostly get done with our sprint task and all critical and major bugs.
So what does tester two in the first week of the sprint?
Well, There are always things to test. We have testing tasks in the backlog, that may include some exploratory testing. Many people don't value Exploratory testing but that is extremely important to build quality products. Good testers create task for themselves and find the possibilities where things go wrong and test them.
Hope that helps!
