I can spend a solid hour in the sun before I start to burn, assuming I have a hat on my head.
I've also noticed some correlation between my diet and how fast I burn. Whenever my metabolism is running at/near ketogenesis, it seems like it takes a lot longer for the sun to affect me.
I experienced similar reaction when i was doing keto. It became far more difficult to burn that summer
Just so you know, burning & cancer isn't the only reason to avoid strong sunlight - it also causes skin aging (i.e. wrinkles). If you don't want to be super wrinkly when you're old, wear suncream!
Yes, some stuff make your skin more sensitive to light than others. For example, you should avoid vitamin A creams and some herbs during summer because they increase risk of permanent skin discoloration.