Nature tends to repeat itself, and that means periodic phenomenon is found in many places. One example would be a heart beating. It repeats itself over and over to pump blood throughout the body. The period of the heartbeat varies, so it's not going to be one specific number.
One online source I found states "It beats or contracts about 70 times per minute". So that would make the period 60/70 = 0.86 seconds approximately. Every 0.86 seconds or so, the heart completes a full cycle. This is when the person is at rest and not doing any strenuous physical activity (in which case, the period would be much shorter).
Another biological example would be the act of breathing. This is also dependent on if the person is at rest or if they're doing some physical activity such as jogging.
As for an example that's outside the personal element, you could go for something like the motion of the tides. These periodic events are due to the position of the moon, which is also a periodic phenomenon. More broadly, the motion of the planets overall is periodic. However, there may be slight changes here and there as time goes on due to imperfections.
