As soon as you notice your tattoo starting to fade, then it might be time for a touch-up. When you initially get your tattoo done, ask the artist if they have any deals on free or discounted touch-ups after you are tattooed.
As soon as you have stopped pealing in the area you should be fully healed. It's always a good idea to wait 2 weeks to insure that it has been fully healed before you can recolor it. Just make sure you are not still in the healing process.