Seat maps aren't a very good indicator of actual availability. It doesn't matter if you think the flight is empty. You need TWO seats in the required inventory class.

Depending on when you want to travel and how much time you have, it can be worthwhile to check often, but really frequent changes might be more likely closer to the travel date.

If there's a big special event at your destination around your travel dates, don't expect lower inventory classes to be available even far in advance and even if the flights are empty. DL knows that it can sell those seats for high prices.