30mins is the cutoff time, ideally even with a teaser you want to go through the five stages of a story so while it is acceptable you may want to modify your teaser to go through all the stages:
Exposition: Setting the scene.
Rising action: Building the tension.
Climax: The exciting bit.
Falling action: Tidying up loose ends.
Resolution: Ending the story.
You dont have to share the ending of the story, just make sure you leave them on a note were some resolution has occurred so that there is some satisfaction for finishing the story. Many times these show up in the form of story arcs within the story.