The Boss' Coffee

That morning it was Lilith's turn to bring the old fat head his morning coffee. Every morning, somebody drew the short straw and that morning, the probability laws selected Lilith. Everybody, even the sycophants, hated coffee duty because the boss demanded that his coffee be exactly 70 degrees Celsius when delivered. Not a degree lower or higher, thank you very much. So, each morning, some poor chump had to pour coffee from the machine, which dispensed it at 97 degrees C. (Everybody else liked it hot.) Then, said chump had to insert a thermometer and wait until the reading went down to 71 degrees. Then, he/she hastened into the boss' office with the beverage and, in the time it took to deliver it, the temperature decreased one degree, so the boss had coffee at 70 degrees, exactly, and you're welcome very much.
Poor Lilith, though! She poured the coffee, inserted the thermometer and four minutes, later, when the thermometer read 92 degrees, it stopped working. She didn't panic, but, instead, she used her knowledge of Newton's Law of Cooling. She started the stop watch on her cell phone at once. She then noted that the ambient temperature was 23 degrees C. How many minutes more did she have to wait before she brought the coffee to her miserable tyrant of a boss?