A. When you think you’re getting close, pour the reduction into a Pyrex measuring cup. If it measures 1 1/2 cups you’re done. If not, pour it back into the saucepan and keep at it. And by the way, with the coconut milk be careful not to reduce below 1 1/2 cups, as the mixture will start to separate.
As to how long it takes, it so depends on the temperature at which you’re reducing it. When it looks like it’s reduced by about half, measure it and take it from there