First things first, in order to give your hair the proper care and find out how often you should be washing your hair, you need to know what hair type you have in the same way you'd need to know your skin type is to find the best products.
Check the density (how much hair you have)
Measure the circumference of your ponytail - if the result is more than 3 inches, your hair has high density, if it's 2-3 inches then it has medium density, and if it's below 2 inches then it has low density. - If your hair is too short to put it into a ponytail, take a look in the mirror - if you can see your scalp at all without having to part your hair, then it's likely that your hair has low density.
Check the thickness (how thick each individual strand of hair is)
Pluck one strand of hair out from the middle of your head and compare it with a piece of thread - if the strand of hair is the same thickness, or thicker, than the thread it's likely you have thick hair, but if it's narrower then you have thin hair.
Determine the porosity (how well your hair retains moisture)
Using the same strand of hair, float your hair in a large bowl of water - if the strand floats, your hair has low porosity meaning your hair retains less moisture, if the strand sinks after a while your hair has medium porosity, and if the strand sinks to the bottom quickly you have high porosity meaning it retains moisture well.