No it's not, time is the time since the begining of the layout, so yeah, it will be equal to 7 seconds only once... 7 seconds after you launch the layout.
What you need to do is :
(event 33) Sack: On Collision with Brille
-> Sack: Set Stunned to True
-> Sack: Flash: Flash 0.1 off for 3.0 seconds
-> System: Wait 3.0 seconds
-> Sack: Set Stunned to False
All actions that are put under the Wait action (in the same event) will be triggered after the number of seconds specified.