According to an article from newscientist.com it can take up to 4000 years for rainforests to fully regenerate.
The Federal University of Paraná studied different parts of the Brazilian Atlantic forest. Sadly this rainforest used to cover 1.2 million square kilometers, but only a measly 100,000 square kilometers of forest remain today. Researchers at the university looked at different parts of the rainforests to see how is was able to cope with human activity.
The good news is that certain aspects of this forest could recover in just 65 years, but it will probably take up to 4000 years for it to fully regenerate.
The rainforests need protection now!

Related to this news is this series of images on boston.com of Amazon Indians protesting plans to build a hydroelectric dam that will destroy their living grounds. These images are truly remarkable and we should really support these natives in their fight against the big corporations/government.