By our reckoning, you could spend 15 hours of lesson time on it (or more) if you wanted to, but most of you probably won’t!
The key thing is giving the students the chance to interact with the scientists, so definitely try to have at least two sessions online – a live chat and a session for submitting questions. And at least a short follow up on these to see the answers and how the voting is progressing.
But remember, if time is short you can set visiting the site as homework. In I’m a Scientist in August 2012 over 50% of students who answered our survey logged on to the site from home, in their own time, to see how it was going.
If you want to spend a lot longer there are 6 hours of lesson plans, plus debate kits.