Why do we use retainers?
As long as you live, your mouth is in a constant state of adaptation and change. This is true before braces, certainly during braces, and forever after braces. This does NOT mean that your mouth will "go back" to the way it was.
But it will certainly change.
If you do nothing, you will have no control over the change-it will happen according to Mother Nature's whim.
If you use retainers, you will have control over change. It is just like exercise. Your body is going to change over time. You can do nothing and see what happens, or you can watch what you eat, exercise and exert some control over what happens. But something will happen.
How long should I use retainers?
If you really want to influence change in your mouth, the answer is forever. Your mouth is always changing and adapting.
When does my mouth solidify and stop changing?
Please read the previous two paragraphs.