I think the response to give you is, "It depends." My first thought would be to make them focused on different strategies as you mention. However, if you began a comprehension lesson, let's say, and felt like your students needed a bit more work to build understanding, you might continue that into the next lesson time.
It's important that your focus lessons are related to your students' needs, so you can't plan them too far ahead. Yes, you can have a plan with where you think things are going, but I've found that some of my most effective lessons are when I notice a strategy needed by several students as I'm conferring, and I change my next focus lesson to address that.
