Skin care: how long does sunscreen last?

At most 2 hours. Although the spf rating of a sunscreen tells you how effective it is during use, it is important to reapply frequently and in adequate amount. Sunscreen should be reapplied every 2 hours to maintain efficacy, as factors such as sweating, water exposure, and physical removal can decrease its effect and lead to burns and skin cancer risk.
Varies by type. This will vary according to the product as well as the activities of the individual ( sweating, exercise, bathing/swimming). Appropriate instructions are usually provided with the product .