User Settings: General > General > Notification
Under Notification you can define the appearance of the Across messages you receive from other users or from the system (e.g. when a task has been completed). You can select whether Across messages are displayed as pop-ups and if so, for how long.
You can also specify whether pop-up messages should only be shown when the user has carried out a certain action in Across (e.g. switching to another module) and whether the user or system message should be automatically regarded as "read", meaning that the icon () will no longer be shown.