If you're asking about the earliest primates:
* The first primates evolved around 55 million years ago during the Eocene epoch. These early primates were small, arboreal creatures, and the exact lineage that eventually led to monkeys is still debated.
If you're specifically talking about monkeys as we know them today (including New World monkeys and Old World monkeys):
* The evolutionary split between New World and Old World monkeys occurred around 40 million years ago.
* Old World monkeys evolved in Africa and later spread to Asia.
* New World monkeys evolved in South America.
So, while the first primates appeared around 55 million years ago, monkeys as we know them today have been around for roughly 40 million years.