A scene in general, or a scene for Prep?

For Prep, I think you have to spend a minimum of 15 minutes working on the prompt for the day, but there's no upper limit. It could be as long as you need it to be.

In general... the same rules apply. It can be however long you need it to be. I've seen some scenes that spanned a whole chapter and others that filled less than a third of a page. It depends on what you're going for. As soon as your scene has done what you need it to do, it's finished.

I'm not sure if this was helpful at all... ^^;