I apply Tri-Luma to the lower 1/3 of my face only, because that's where I have hyperpigmentation. I do apply it to the whole area though, because it doesn't make sense to me to try to apply only to the darker areas. It works wonderfully for me, but as I said, I am very, very careful about sun exposure.
After the initial couple months of daily use, I did reduce to about once per week, as "maintenance", as someone mentioned above, but occasionally, I need to do a week or two of every day application to get back to clear. Hyperpigmentation always wants to come back, so it's a constant battle.