How much weight can one half inch drywall hold with a drywall anchor in the ceiling?

If there's any question about whether or not a drywall anchor will hold on the ceiling, the best thing to do would be to find a ceiling joist and use a long screw to go into solid wood. (Use a stud finder to locate the joist.) Hanging on a ceiling is more dangerous than hanging on a wall since things that fall can fall on people or pets.

If you HAVE to hang just in drywall, use the strongest anchor you can get (i.e. one that expands like a butterfly after insertion, or the type that flares out like a flower when you tighten it). Your local hardware store can help with what's a reasonable weight limit.