As long as you want!
It also depends what community you are posting to, make sure you read their posting rules!
As a good guide, try to do between 500 -600 words and make sure it your words, not copy pasta from elsewhere, don't use AI! If you use pictures then make sure they are yours, or they are free to use , from pixabay for example. Remember you are earning from a post, so it is commercial. Copyright is a thing.