Tim and Julie got it. though the reasoning is slightly different.
You would have to wait 90 minutes between 12:15 and 1:45.
Once you had heard seven single chimes, you would know that the next chime would be two chimes for 2 o'clock.
The reasoning being that you wouldn't have to hear the 2 o'clock chime because you would have heard 7 single chimes, the most single chimes in a row possible.