The formation time of the Amazon forest cannot be accurately given because it is a long natural process. The South American continent where the Amazon forest is located has experienced changes in geology, climate and other factors in the past millions of years, and these changes have jointly contributed to the formation of the Amazon forest.

Specifically, the evolution of the Amazon forest can be divided into the following stages:

1. About 180 million years ago, the South American continent began to split, and the Amazon region gradually sank, forming a vast ocean basin.

2. About 150 million years ago, the South American continent stopped splitting, and the sea water began to gradually withdraw from the Amazon region, leaving an inland sea basin.

3. About 65 million years ago, the crustal movement of India caused the eastern part of the South American continent to rise, and the Amazon region began to receive sediment accumulation.

4. About 5 million years ago, the western part of the South American continent rose, the Andes Mountains were formed, the moisture in the Amazon region began to increase, and the forest began to gradually develop.

5. About 1 million years ago, the Earth entered an ice age, which affected the Amazon region and reduced the forest area.

6. About 100,000 years ago, the ice age ended, the Amazon forest began to recover, and gradually developed into its current size.

In summary, the formation process of the Amazon forest took hundreds of millions of years. However, we need to note that this is a long natural evolution process, not an event in a specific year. Within the scope of human history, the evolution process of the Amazon forest is relatively stable, but due to the impact of human activities on it, such as deforestation and climate change, the future of the Amazon forest still faces many challenges.