One of the things I tell my higher level students is that if they don't work on the details of their techniques, and only practice it the same way they practiced it as a white belt, then they will get faster and stronger at the white belt technique, but it will still be a white belt technique.
In Taekwondo, with a single reverse punch, there may be dozens of details I'm looking at for an advanced student, but only a few details I'm looking at for a white belt. If I throw every single piece of information at a student at once it will overwhelm them and they won't know what to focus on (I've done this before and I've gotten several lectures from my Master on this), so we teach only what is needed to learn the basic motion of the technique and then improve it over time.
So if you practice 15 minutes a day, you will maintain the memory of what you've learned, but it won't help you improve your technique. For that, you need actual feedback from an instructor.