There's a huge difference between shoulder and boob length in terms of long-hair experience, but either way it sounds like she has agreed to spend up to 3 years growing her hair to waist length for you. That's amazing and you're very lucky.

Worry about butt/knee length down the road. That takes a different level of emotional commitment from the woman. If you stay together long term, maybe you can convince her to grow it closer to her butt or classic length for the wedding, as that's the moment where so many women have fairytale princess dreams.

Enjoy the grow out; compliment her on her beauty, her devotion to you, and her hair. Help take care of it if you can. Encourage her to wear it in different styles including updos if you want her to go longer than waist length in the future, because wearing it down most of the times at those lengths is impractical.