There are many misconceptions with sunscreen. You have those people who think that getting the highest SPF will protect them better than a lower SPF. Then, there are those who won't use sunscreen with a high SPF because they think they won't get a tan.
Here are the facts.
Higher SPF's aren't always necessary. You need to look at a couple of things when you are shopping for sunscreen. First, how light is your skin tone? Those with lighter skin will usually burn faster than those with darker or olive skin. If your skin starts to turn red fast, you'll want a higher SPF.
But what does the SPF rating really mean?
SPF stands for Sun Protection Factor. To figure out how long you are safe from the sun (at least the UVB rays) you need to do a little math. Take the number of SPF and multiply it by 10. That is the time that if you were under perfect conditions, you'd be safe from the sun's rays.
For example: SPF 20 x 10 = 200 minutes of sun protection
Perfect conditions mean that you aren't in water or sweating, two things that usually go hand in hand with the sun. This also doesn't reflect sunscreen that can rub off from your clothing.
Because you want to be safe from the sun, I'd recommend applying every 1-2 hours, no matter the SPF.