When were the dreamtime stories invented?

This is impossible to answer on many counts.

First, the stories were not invented, but grew over many tens of thousands of years as the people adapted and conformed to their continent.

Second. Aboriginal people have an oral, not written, tradition, so we can't point to written references as we can with some ancient, but younger cultures such as China.

Third. The Dreamtime (it should always have a capital D) stories exist on several levels. One level is secret and sacred. It regulate the law, including the way the country is sung, and is not fully divulged to anyone without the power and knowledge to cope with the secrets involved.

Another level is the parts of the stories that are told to children and the non-initiated. The stories explain why animals behave as they do and contain moral lessons essential to survival of both the individual and society.

Perhaps a disappointing answer, but it answers the question if a little briefly.