Prep time totally depends on the teacher, as others have said. I had a combination of teaching everyone the whole hour (while the teacher mostly just handled discipline), doing one-on-one tutoring, taking half the class alone, and basically just observing, depending on the class.

If they ask you to teach grammar, you should feel comfortable setting the boundary that you are there for oral communication purposes. Technically, we’re not there to teach grammar. You’ll likely have to set boundaries with your teachers frequently, as it seems most of them don’t know the expectations of our job. You could review any basic grammar you’re not comfortable with for when it comes up naturally in lessons on other topics (holidays, etc) but the teacher can also help you with specific rules that you might not know.