It takes approximately 2 hours for semi-gloss paint to dry, depending on the type of paint used. While paint may feel dry to the touch after about an hour, most paint manufacturers recommend waiting at least 2 hours or longer before applying a second coat.
Although paint feels dry after several hours, it's not fully dry until it's cured, which is a different thing. It takes several days, and sometimes weeks, for paint to cure. When paint cures, it reaches it's maximum hardness and is considered 100 percent dry. Until the paint is cured, it's very easy to scratch and mark the surface.