Complex sentence: According to Sandberg, to-do lists "aren't the key to productivity they're cracked up to be" and reports an estimate that "30% of listers spend more time managing their lists than (they do) completing what's on them".
Split into simpler sentences: According to Sandberg, to-do lists "aren't the key to productivity (that) they're cracked up to be". He reports an estimate that "30% of listers spend more time managing their lists than (they do) completing what's on them".