Two things matter most: enough product and consistency. For the face, aim for roughly a quarter-teaspoon (the "two-finger" guideline is a rough proxy) — most people apply far less than the amount sunscreens are tested at, which quietly lowers the real SPF. Reapply about every two hours of meaningful sun exposure, and after swimming or sweating.
On the payoff: daily sunscreen use is the step with an actual randomized trial behind the anti-aging claim. In the Nambour trial, adults using sunscreen daily showed less visible skin aging over 4.5 years than a "use as you like" group (https://doi.org/10.7326/0003-4819-158-11-201306040-00002). The variable that mattered was consistent daily use — not an exotic ingredient. Indoors all day with little window light? Reapplication matters much less; the daily habit is the win.