I empty it every 4ish hours, or sometimes more - about the same regularity I'd change tampons with. It's never been more than a quarter full (though I do have quite light periods). I guess in theory I could leave it for much longer, but this seems like an Inherently Bad Idea.

Yes, you only sterilise it once, and rinse it as you go. The instructions also recommend you wash it with plain soap and hot water occasionally while it's in use, so I tend to do that while I'm in the shower.