Any sun protection has its limit. The factor refers to how many times your natural protection, so if for example you burn after five minutes of strong sun exposure, a factor 30 sunscreen will protect you for 30 x five minutes: 150 minutes or two and a half hours. If you burn after ten minutes, factor 30 will give you 300 minutes or five hours of protection.
If you burn naturally after five minutes and use factor 50, you will have 250 minutes, or just over four hours of protection.
And that protection relies upon it being properly applied and not being rubbed or washed off along the way. The protection starts from when you go into the sun and the time limit isn't renewed by reapplying, that's your one lot of sun protection. You can't reapply every ten minutes and get an extra ten minutes in the sun.
Many people do not use nearly enough sun cream/spray/oil to benefit from proper protection, you have to be VERY generous with your quantities. You should also apply about 30 minutes before going in the sun and allow to dry thoroughly. To maintain your protection within the stated time limit, you also need to reapply after rubbing the skin or being in water.