all year, even if you're just hiding in your house. UV is present ALL THE TIME, and they can go through windows to reach inside your house. those are great deals of UV so don't underestimate it. even if you can't see the sun, remember that UV aren't visible - so UV would still be there even if you can barely see sunlight.
if i need to be out in the summer for the whole day, i'll try to use spf50 broad spectrum and reapply 2-3 times. if i just stay inside or hardly ever go out when there's sunlight, or if it's in the winter, i'll apply it once a day and that should be good enough. or you can also use spf30 for when you're inside. but please be sure to use sunscreen cuz it's the most important step of skincare. healthy tan is sexy but sunburned, hyperpigmentation, sun-damaged aging tan is not. you can still get tan with sunscreen on, but it's gonna be a healthier tan and not damage your skin or fasten aging rate (it'll take longer to achieve the tan you want but think about your health first). you can also use products to boost tanning (i'm not an expert in this field, but you can go on youtube and watch videos about tanning process of some beauty blogger, all of them use sunscreen)