Do you apply sunscreen 365 days a year?

They say it's a sunscreen that you have to apply 365 days a year.

To be honest, when I'm indoors, I close the blackout curtains and don't really apply sunscreen. I also don't apply it when it rains. I'm wondering if there would be any issues even if I closed the curtains, since they aren't 100% blackout. Also, wouldn't there be almost no UV rays when it rains? Should I still apply it then? Since sunscreen requires thorough removal, applying it every day feels like a hassle.