Brass does tarnish and the rate of tarnishing varies depending on lots of different things that are mostly out of our control such as the amount of sulphur in the environment.
You can purchase a commercial brass cleaner like for brass household objects. There are lots of other easy home methods too.
If you google "How to clean brass jewelry" the number one easy method is soaking or brushing the jewelry in ketchup.
Apply it with a toothbrush or soft cloth, then rinse thoroughly. Avoid getting ketchup on the same types of stones as should not be used with chemical cleaners.
Create a mixture of lemon juice, baking powder, and water. Put some on a toothbrush and gently go over your brass jewelry.