Most skincare advice is a matter of taste. This one is not. Daily sunscreen is the single best-evidenced thing you can do for how your skin ages, and the gap between it and everything else is not close.
What the evidence actually says
The study people cite is a randomised trial in Nadi Beach, Queensland, that ran for four and a half years. One group used sunscreen daily. The other used it when they felt like it. At the end, the daily group showed measurably less photoageing. Not a small difference, and not a difference anybody could have argued away, because people were assigned to their group rather than choosing it.
Follow-up work on the same population found the daily group also developed fewer skin cancers, and that the effect persisted years after the trial ended.
The part nobody mentions
Almost everyone underapplies. The amount used to establish an SPF rating is two milligrams per square centimetre, which for a face is roughly a quarter of a teaspoon, or the length of your first two fingers squeezed out.
Most people apply about a third of that. An SPF 50 applied at a third of the tested amount does not give you SPF 17, because the relationship is not linear, but it does give you meaningfully less than the number on the bottle.
The practical version: use more than feels reasonable. If it feels like a lot, it is probably about right.
SPF 30 or SPF 50
SPF 30 blocks about 97 percent of UVB. SPF 50 blocks about 98 percent. On paper that is nearly nothing. In practice the higher number gives you headroom for the fact that you will underapply, so if the two feel the same on your skin, take the higher one.
What matters more than the number is whether you like it enough to wear it every day. The best sunscreen is the boring one you reach for without thinking.
Cloudy days, and being indoors
UVA passes through cloud and through window glass. UVB largely does not. So on an overcast day you are still getting the wavelength most associated with photoageing, and sitting by a window at work is not the neutral situation it feels like.
This is not a reason for anxiety. It is a reason to make it part of getting dressed rather than a decision you make each morning based on the weather.