One application can give you up to 8 hours protection, as long as you follow the guidelines.

"SPF" = The Sun Protection factor. It it takes 10 minutes for your unprotected skin to start turning red, using an SPF50 sunscreen theoretically* prevents reddening 50 times longer-about eight hours. The SPF is tested indoors, on volunteers, using a light source that mimics noon-time sunlight. The SPF is a measure of UVB protection which is what causes burning. UVA causes aging - this is a broad spectrum product and protects your skin from both UVB and UVA. The minimum protection for UVA in Europe is just under 3 stars, this is a 4 star product.
*Every one's skin is different and the sun varies in intensity - take care in the sun to avoid over exposure.