Unfortunately, I don't know. The CDC only stated that the smallpox vaccine is 85% effective against monkeypox but didn't mention how long (source: https://www.cdc.gov/poxvirus/monkeypox/clinicians/smallpox-vaccine.html).

And I can't seem to find the original data where CDC get that information from.

That said, smallpox vaccine's immunity lasts for 3-5 years and then starts to wane (https://www.health.ny.gov/publications/7022/). Assuming no smallpox immunity means no monkeypox immunity, I'd assume the smallpox vaccine we got decades ago has lost much of its effects.

But until more data comes out, my assumption could be wrong.