The point is to keep your mind focused on one still thought. Letting your mind wander around won't do much. Complexity of the thought should match your ability.

The point is to keep your mind focused on one still thought. Letting your mind wander around won't do much. Complexity of the thought should match your ability. Lam-rim says that more short meditations is better than one long meditation. Ideally, one would meditate 16 times a day. And again, the whole point is to stay focused on one thought.