There is not a specific interval for changing artwork. If you really like a piece you can keep it in a prominent place forever. If you made a major change in room color, the artwork might look out of place, too bold or too pale. You can switch art from room to room as long as the subject fits the mood of the room. Floral and landscape art can fit most rooms. Still life of fruits might work in the living and dining, but look silly in the bedroom. It really comes down to how it makes you feel.
The artwork need to match the room. Anyway you can change it according to your like, there is no specific time for this thing.
It depends. I think if your artwork is hand-painting painting, you may change it if it is faded. Recommend a painting reproduction gallery for you: http://www.artisoo.com/
Whenever you feel like it.
never
