Actually, advertisers don't pay much if any attention to how long a listener spends with a station each day or week. What they look at in the agency versions of the Nielsen software is the number of listeners that hear each spot.
"Reach & Frequency", the math that gets you both per-spot impact average and total number of "hears" of each spot based on cume, is not implemented much any more. Just AQH persons per spot.
So, in a simplification, I am saying that they don't care how long you listen... just if you heard a spot and how many others did, too.