Whenever you will be outdoors in daylight for more than about 15 minutes. But also be aware that UVA (which is the main cause of ageing) can penetrate through glass, so you will still be exposed sitting in a car.
Ideally year round as the UVA levels are fairly consistent year round and even when there is cloud cover. UVB causes burning and these levels increase with the intensity of the sun.
Sunscreen should ideally be reapplied every 2 hours if you are outdoors to maintain the protection. This is especially important on hot sunny days as the risk of burning is higher due to increased UVB.
My personal practice is to apply Altruist SPF 30 as a post shave moisturiser every morning and then extend the coverage to include the rest of my face, my ears and my neck.