|The view from Notre Dame in Marseille, France|
Understanding Sun Protection Factor:"If your skin would normally burn after 10 minutes in the sun, applying an SPF 15 sunscreen would allow you to stay in the sun without burning for approximately 150 minutes (a factor of 15 times longer) " - Badger Balm Site.
A safe bet in my opinion is an SPF of 30, labeled broad spectrum UVA/UVB. UVA rays cause long-term damage, like age spots, and UVB rays cause the immediate sun damage, which usually results in a burn. There is also a difference in the actual protective UVA ingredients in sunscreens, which can be categorized into "mineral" or "synthetic" protection. You can see more on "what the sun is doing to our skin" in a fun graphic here, on Paula's Choice.