How do I know when my wine is properly reduced?
The goal is to evaporate alcohol and concentrate flavor.  If adding wine to a mirepoix, or sofrito...some sort of early stage aromatic vegetable... reduce the wine almost until the pan goes dry, but not dry enough to cause sticking or burning.  The flavor will be absorbed into the vegetable and you are good to go.  If there is another instance of wine addition that you are considering, please specify.