How long do raised scars take to flatten and fade?

It depends: In general, the worse the injury, the worse the scar and the longer it takes to fade. That is why wounds must be treated gently and encouraged to heal quickly. Older people will find that their scars fade faster than younger people (it's one advantage to age); darker-skinned people tend to take longer than fairer people. Keep all scars out of sunlight until they fade as uv worsens outcomes.