i'd say two hours. That would be assuming you just heard the last chime at 12:00 (I guess maybe you were just walking in the door or something). Then it would chime once at 12;15, 12:30, 12:45, 1;00, 1:15, 1:30, and 1:45. It wouldn't be until it chimes twice at 2:00 that you'd know that the first chime you heard was the last chime at 12:00 and not the 12:15 chime.
Johnathan
Johnathan