I get asked about dark spots more than almost anything else,  so I want to answer honestly, the way I'd answer a friend.

"Does my SPF actually work on dark spots, or is it just protection?"
Both, but not in the way people expect. SPF doesn't fade what's already there, it stops existing spots from getting darker and new ones from forming. Think of it as holding the line, not reversing it. The fading work has to come from somewhere else, like Vitamin C. 

"How long will it actually take to see mine fade?"
Real, honest timelines are about 8 to 12 weeks at minimum, depending on how deep the pigment sits. 

"Vitamin C Serum or Retinol Serum — which one is actually for dark spots?"
Vitamin C works on the pigment itself. Retinol works on cell turnover, which helps pigment surface and shed faster. Use them together. Vitamin C in the morning, Retinol at night.They're doing two different jobs, for one shared outcome. 

"Will they ever fully go away, or do they come back?"
Some fade completely, but some fade and stay lighter for good, as long as you protect the skin that's underneath. What we can promise is visible improvement, not permanence. Anyone who promises you "gone forever" isn't being honest with you.

— 

Valérie Grandury