No she wasn't...
Get your facts straight.
Count Two, Aggravated Child Abuse: The maximum penalty for aggravated child is 15 years in prison. The estimated minimum sentence for this charge and other charges which might result in conviction along with this count would be 12 years, 1 month.
http://www.cfnews13.com/article/news/2011/july/273571/Casey-Anthony:-The-charges-against-her