@function makelongshadow($color) {
  $val: 0px 0px $color;
  @for $i from 1 through 300 {
    $val: #{$val}, #{$i}px #{$i}px #{$color};
  }
  @return $val;
}

eg.

See the Pen ZNYNgb by blwoosky (@blwoosky) on CodePen.1449