BuddyPress.org

When it comes to hiding stuff… I’m always a fan of the ultra-low-tech CSS approach:

span.time-since {display: none;}

There are two instances of this class in the screen.file by the way

.activity-list li span.time-since {}

.widget_bp_activity_widget span.time-since {}

You may want to use those classes to target where you want ‘time since’ to show or not show.