How long exactly does permanent black hair dye last for? The one I used was 24 washes

What happens will it just fade away and my natural hair colour will come back?
This was my first time dyeing my hair, my natural colour is blondie brown

I believe you answered your own question, 24 washes.

However it varies on how you wash your hair. In my experience, they usually start fading after about a month with daily showers.
In most cases, your hair will become it's real color, eventually. However, some people's hair "stain" and it stays a mix between the natural and the dye. They have to wait until their hair grows out again to their own color.