Moira, I believe it is all day. I do not think there is a particular time set out for Joey's guest Blog. From other blogs I have seen. The host of the site will type up an introduction and the guest will give you a taste of what ever it is they are talking about. Might even ask the readers a question or two and then we readers, read and comment. It sometimes a 24 hour process.
If you can check out the blog through out the day.
I hope this is helpful. I hope I am not off the mark with what I typed. If I am...sorry :-(