Really depends on how much you practice.

I feel like a style is constantly evolving, and as an artist I think you're always looking for ways to improve, so it's never perfect.

A year or two is a good estimate. Sketching and experimenting with all kinds of different ideas is really the way to come with a unique and interesting style.

Also preserve the letters, this is graffiti, it's important to maintain structure, but get weird don't let that constrain you.

Hope that helps