I don't think there is any right time to use graduated colour filters. There is nothing they do which you can't achieve in post and you have little chance of undoing any effect they introduce later.
ND's are very useful to maintain your shutter speed at or close to twice the frame rate for video. Obviously the darker ones are used in brighter conditions. They are of little use in still shooting unless you are aiming for slower shutter speeds to introduce some creative blur (seascapes, waterfalls etc).
Polarising filters are good for arjening skies, introducing saturation and reducing reflections. Most of this effect can be realised in post (with the exception of reflection reduction) and you will often find skies vary greatly in overall brightness due to the wide FOV of the lens (can look very off and exceptionally difficult to correct).
UV is a good general purpose leave on to protect the lens when not using other filters.