Go to notification list > edit notification > Data & Timing tab  > Timing rules

If you want to change the timing for the appearance of the notifications then click on the notifications list and then choose edit notification. Then choose the Data and Timing tab and then timing rules. Here you will see four fields that can be customized as per your choice.

Initial delay time: After this time first notification will be displayed:

The Initial Delay Time is the time after which the first notification appears on the screen.

Display notification time: Notification will display for this time duration:

The second field is of Display Notification Time which is the time period of visibility of the notification.

Apply fix delay time between the notifications: Next notification will display after this time duration:

The third field is, Apply Fixed Delay Time Between Notification. Which is the time interval between notifications.

Randomize delay: Turn this option ON to randomize the time gap between notifications.
The fourth and the last field is Randomize Delay, if you want to set random time period between notifications.