It is up to you to decide how long before an activity needs to be booked. In other words: Until when is a reservation possible before the "D" day of the activity?
While it is quite common not to establish a notice (for an experience that requires no preparation), it may be that certain types of activities require it... For example, an activity offering privileged access to a place to take pictures of, may require the mentor to submit an access request at least a week in advance.
The notice period can also be chosen for comfort reasons. When your activity is instantly bookable, establishing a notice will allow you to plan your days a little in advance. A notice of at least 24 hours will also prevent a customer from being added to your group at the last minute (in the case of an instant booking).