Hello Mai:
Galkyd dries generally overnight or in about 2 days depending on how much you use and the particular color.
If you want to paint direct over a glazed passage, I might recommend that you consider wet sanding the surface first with a 400 or 600 grit automotive sandpaper (this is the black stuff). That will help to remove the bumps and it will abraid the surface which will help with layer adhesion.
As for painting over a glaze that you did not want, you could certainly scrape or sand it out. You could also paint direct over the glaze and just obliterate it. In a hundred years or so, as the top layers turn more translucent, there "may" be a slight tinting of the color from the glaze below -again I say "may" as it depends on the color intensity of the glaze and the composition of the top layers as to how translucent they will become over time.
Hope that helps.
