I recently bleach my skin, I was a light-brown color and now I'm a fair tan color. Do have to keep use whitening products to keep my color?

Bleaching your skin is actually very harsh on your skin. But being that you started you will have to maintain this new skin tone. I would recommend something mild so it does not do anymore damage to your skin. Something without the hydroquinone, lemon juice will work. Also Dermalogica has products to help lighten skin though its mostly for the face, and if you have done your whole body that could get expensive. Here is a link for Dermalogica:
http://www.dermalogica.com/us/
Hope this helped :)

Will melanin permanently reduce a little from bleached skin when the skin go back to normal? How long does bleached skin last? And are whitening pills permanent?

Whitening stops the production of melanin. You have altered the skins configuration and stopped the production of melanin. Skin in now affected by the suns rays, it will reoxidise leading to more pigmentation, or possibly hyperpigmentation/hypopigmentation and premature aging. This also happens if you use any sort of peels on the skin. I believe that the severity of the damage already done to the skin and any possible chance of seeing normalcy, will depend on the bodies ability to recoup in attempt to continue to produce melanin in an even tone, though it is a rare chance it is not impossible for the body do. Here are some sits that inform you of the effects:

http://docgerrytan.com/2006/09/26/the-harmful-effects-of-skin-whitening-creams/
http://www.nytimes.com/2010/01/16/health/16skin.html?pagewanted=all
http://www.ehow.com/about_5106404_side-effects-skin-bleaching-creams.html