Just enough for a brand name and a brief slogan. The rest should be art.

I seen some in Mexico that have a full description of what the business do what they sell, directions, who works there, as if they wanted to get the most information from the billboard, but finished being too crowdy, and going in the car, you really have to stop and finish the "newspaper billboard"

pix (of models...)+ tag-line+ name of product
Pix (of product) + tag-line
I often imagine a huge pix of Pepsi with the tag-line of only one or two words.