"The sooner the better" in terms of treating amblyopia. The longer a parent delays treatment the worse the amblyopia becomes and the harder it is to cure the amblyopia.
In general, the main treatment for amblyopia is occlusion therapy, in which the child has to wear a patch over the stronger eye and force him/her to use of the amblyopic (lazy) eye. Occlusion treatment is not an easy treatment for the child or family or for school personnel. Often, PARENTS will fail with the occlusion therapy - They'll say something to the effect that " he just won't leave the patch on." If the parents fail with occlusion, the child is often left with only one good eye. Typically, an adhesive patch is used for treating amblyopia but sometimes a patch can be placed over the child's glasses, if he/she wears glasses.
If the child's vision isn't too bad (generally better than 20/100 visual acuity), an alternative to patching may be to use eye drops in the stronger eye. The eye drops will dilate the pupil and cause blurred vision in the stronger eye. With the stronger eye blurred, the amblyopic eye has a better chance of regaining vision. Similar to occlusion therapy, the dilating drops have to be used for a long time, sometimes 6 months or more, and such "penalization" therapy only works if the amblyopia is only mild to moderate.
Another important aspect of therapy for amblyopia is the correction of the initial cause of the amblyopia. For example, if the child is anisometropic - has a difference in refraction between the two eyes, then the child must be placed in corrective glasses. Sometimes the glasses alone will cure the amblyopia. If the child has a cataract, the cataract must be removed by surgery and a corrective lens or contact given. If the child is crossed-eyed (esotropic), then once the amblyopia is cured the child may need eye muscle surgery, glasses with polycarbonate lenses, or orthoptic (eye muscle) therapy to correct the eye misalignment.