What is the treatment for skin become dark post bleach. reason being bleach being put up for more than 30 mnts?

Needs to be seen: This is chemical burn with hyper pigmentation. To even skin color an individualized approach whether prescription cream, laser with microdermabrasion or selected chemical peel at appropriate time.

Hyperpigmentation: Sounds like the bleach caused an irritation, which can lead to temporary darkening (post-inflammatory hyperpigmentation) especially in darker skinned individuals. Protect the area from sun exposure and give it time to resolve.

Onchronosis: Rare condition called onchronosis which may be from using high strength hydroquinone for long periods of time in predisposed individuals. Definitely stop hq if that is what you are using and seek some natural products for bleaching. Skinceuticals makes one called pigment regulator. These don't work as well as hq but may be only option for you. And complete sun protection always!