The oscillation frequency of the balance wheel as it rotates back and forth (to and fro) is referred to in VPH (vibrations per hours) or BPH (beats per hours), which are the same thing. The frequency of oscillations can also be measured in Hz (Hertz), meaning cycles per second.

18,000 VPH/BPH = 2.5Hz
21,600 VPH/BPH = 3Hz
28,800 VPH/BPH = 4Hz
36,000 VPH/BPH = 5Hz
43,200 VPH/BPH = 6Hz
72,000 VPH/BPH = 10Hz