That depends on when the parents apply after they've adopted the child. Furthermore, it varies a great deal from one country to the next, largely because of the complexity. In the case of China, for instance, the processing time is under one month. So it may take a few weeks between when the application is received at the embassy and when the citizenship is granted. However, parents can begin the process two years in advance.
On December 13th, 2010. See this statement in context.