Who long is too long to slow cook chicken in crock pot?

who long is too long to slow cook chicken in crock pot? I normally cook it on low for about 8hr which comes out find. If I am rushed I cook it on a high temp for 5-6hr, but even with water it is dry and over cooked. I find that during the day I am too busy and find it easier cooking it at night. I wake up early and it's cooked. How long is too long and does it go bad or rotten? I was told by a friend that with using this method by the time it's done it goes bad. is that true?