Page "Reciprocity (photography)" Paragraph 11

For example, for a given film, if a light meter indicates a required EV of 5 and the photographer sets the aperture to f / 11, then ordinarily a 4 second exposure would be required ; a reciprocity correction factor of 1. 5 would require the exposure to be extended to 6 seconds for the same result.

Reciprocity failure generally becomes significant at exposures of longer than about 1 sec for film, and above 30 sec for paper.