Tom, I think what you are reading in help is what N_C_H_josh was pointing to. It's in reference to an Export Video type of GIF. In the help on page 71 of the PDF it says
Screen References - GIF Encoding Settings
Looping
Select this to get your GIF repeat itself endlessly.
There is no mention of selecting a duration.
I just played with an animated GIF. While it will look indefinitely when I launch it from Windows, PS shows it's length to be 1.3 seconds. PS will not allow me to extend the duration. I copied the GIF to the slide slow 3 times and changed in TRANSITION time to .1 second (lowest allowed). You may want to play around with that approach to get your set duration.
Full disclaimer; I'm relatively new to PS.