How much exposure your skin needs is dependent on how fair your skin is:

Type 1: Light, Celtics, Scots, red-haired and freckles:  5-7 minutes per side

Type 2: Burns easily but tans, Caucasians: 10-15 minutes per side

Type 3:  Sometimes burns, tans gradually, sun-sensitive: 20-30 minutes per side

Type 4: Burns minimally, always tans, minimally sun-sensitive: 40 minutes per side

Type 5:  Rarely burns, sun-insensitive:  50 minutes per side

Type 6: Never burns, deeply pigmented, sun-insensitive: 60 minutes per side.

Modified after a chart from Krispin Sullivan.