It's impossible to give a precise number for how long prokaryotes were the only organisms on Earth. However, we can estimate a timeframe based on current scientific understanding.

Here's what we know:

* Early Earth: The Earth formed roughly 4.5 billion years ago. The earliest evidence of life, in the form of fossilized prokaryotes (specifically stromatolites), dates back to around 3.5 billion years ago.

* Evolution of Eukaryotes: Eukaryotic cells, the type that make up plants, animals, fungi, and protists, evolved much later. The oldest fossils of eukaryotes are about 2.7 billion years old.

* Uncertainty: The exact timeline of early life evolution is still being studied. Some researchers argue that the earliest forms of life were even older than 3.5 billion years, while others suggest the first eukaryotes appeared earlier than 2.7 billion years.

Therefore, we can estimate that prokaryotes were the only life forms on Earth for at least 800 million years, and likely for a significantly longer period. This is a vast stretch of time during which prokaryotes played a crucial role in shaping the Earth's environment and setting the stage for the evolution of more complex life forms.