Sunscreen

If you spend a lot of time in the sun, it is advisable to wear sunscreen every day. It protects your skin from harmful UV Rays, preventing you sun spots and wrinkles in the long run.
However,  make sure that at the end of the day you wash it off really well.

You do not need to change your sunscreen regularly. If it works for you, then it will keep working for you. Unless you don't like the smell or something, there's no real reason to change.

However, if you notice it isn't working that well anymore, it is definitely time to get a new one.