The conversation between human and AI assistant.
[|Human|] How long did it take for ironworking to be commonplace in West Africa following its introduction in Northern Africa?
[|AI|] Ironworking was introduced in Northern Africa around the first millennium BCE and became commonplace in West Africa by 500 BCE. This means it took 500 years (1000-500) for ironworking to become common in West Africa following its introduction in Northern Africa.
[|Human|] 
How long did it take for ironworking to be commonplace in West Africa following its introduction in Northern Africa?