I'd say it depends on what level books/kids you're talking about.
Last year Grant was in level 2/3 student book, and he did the level 2 pack of books. They were pretty easy for him, but a great place to start. We spent 20 min per session on DITHOR. For Level 2/3, you spend the whole time with the student because they're supposed to read out loud to you, and it's reccommended to talk through the workbook page when those come up, too.
This year, I'm planning on Grant needing 30 min for DITHOR. But, I'm thinking I'll only need to be in on 20 min of that? He's going to do student book 2/3 again (less writing) with the 4/5 boy book pack.
I would think if you put aside 30 min of your time 3 times a week, that would be a good guess to start with. Some days you get to do everyone together. On the days you're not all together, you could send the older ones off on their own, and devote the time to younger ones. (I think.

Would you believe that I only read the parts of the plans that I needed last year? I can totally ignore anything that doesn't pertain to me.

So, hopefully someone who's using it with several levels can really help you.

)

Kathleen