It is the multiple of the protection of your own skin. SPF 30 gives you 30 times more protection than no cream at all. I have read that skin experts say the really high SPFs (50+) do not offer any more protection that a SPF 40.
The time in the sun depends on the sun, time of day, the type of skin and what you are doing (and drinking!). However, a SPF 30 at the start of the holiday in short periods, moving to a 10-15 later on when you start to tan is normally ok.
Prolonged exposure to the sun does age your skin quicker and increase the risk of cancer, but taking your top off for 2 weeks a year is unlikely to be a significant risk