The simple answer: when it goes on for more than a page. But we're not here for simple answers.
Some fiction writers deem it necessary to babble endlessly about one piece of scenery, or a character, or a piece of furniture, and doing so is, quite frankly, annoying and boring. No matter how good your description is, if it detracts from the story, it's too much. That's not to say that a good chunk of description can't work; it can, but only if it is good, and if it keeps the story interesting. Most likely your readers are not going to sit through a page of prose describing a chair.
Here are some good ways to know when description is too much:
- Reading your description sections makes you forget what the point of that section was.
- Reading your description sections makes you forget what was happening just prior (or makes you forget the story in general).
- It goes on forever and ever, dragging your character and narrative away from what is supposed to be going on (i.e. the plot). In the event that you're writing a story that doesn't really have a plot, which isn't necessarily a bad thing, then description becomes overbearing if the characters get loses to the reader (or to you).
- Your description drags the story down.
Other opinions welcome. Feel free to leave a comment!