Countdown time of Order Bump works in two ways-
You can use a setting like a static timer. For example you can set a timer for 5  minutes, 5 days, 5 weeks or 5 years. Now the timer will keep on running from that instant time and keep on reducing time, whether the user visits the checkout page or not.
The second method can be like an evergreen timer. It only starts when any user comes on the checkout page. Here checkout means whatever location is setup done from admin end