Cast Iron Kettle

I just thrifted this cast iron kettle for $10. It was quite rusty inside, but I’ve mostly got rid of it. We will be using it for humidifying, as we live in a semi-arid climate.

Just curious if it’s okay to leave water in it or if it’s better to empty it between burns. I estimate we have a fire a couple of times a week.

Thanks in advance! Learning a lot from your expertise reading posts and comments here.