Already a SPF 15 protects against 93.3 % of the UVB radiation. SPF 30 provides a 96.7 % protection. The duplication from SPF 15 to SPF 30 also involves double the concentration of light protective filters while the protection only increases by 3.4 % (96.7 % minus 93.3 %).
The maximum sun exposure depends on the individual skin-inherent protection and the sun protection factor of the applied cream. It is shown in the following figure. Thus, a skin-inherent protection of 10 minutes (purple line) plus application of a cream with SPF 30 results in a maximum sun exposure of 300 minutes (5 hours). The skin-inherent protection varies with the geographical latitude and the altitude of the respective location (rf. Kosmetische Praxis 2010 (2), 10-13).
Please refer to the article "Sun protection: on the efficiency of UV filters" for further details.