Based on an observational study, the smallpox vaccine appeared to be 85% effective at preventing monkeypox. After 25 years, the vaccine effectiveness remained about the same at 80.7%: https://www.pnas.org/doi/full/10.1073/pnas.1005769107

But longer-term data doesn't seem to be available. But based on the on-going monkeypox cases in African countries where the people had gotten the old smallpox vaccine, I'd assume much of the vaccine immunity has waned.