How do I say something on the Mood Panel?

[Aia del Mana]

And how long will it stay there?

[Muratus del Mur]

You can write a short paragraph of text on the mood panel, that will stay there until other entries push it down, or you change it. You can have only one entry at a time, the moodpanel is not a dialogue. 

You can see how long you have to wait till you can edit your entry again, when you try to edit it.