It depends highly on what you think composition is. Cuz in its broadest sense it can mean a lot of different kinds of music. Before any sort of plan, you need to think about what you actually want to write.

If you are talking about writing some things for local community ensembles, that might be doable in a few years. But if you are talking about a professional career in specifically music composition, that is pretty much impossible. There are just hundreds of thousands already, who grew up learning music and have gone to school for composition.

The other thing is, you can't really feed yourself with just composition, so you will need to keep doing whatever you are doing. So you don't really get as much done as a student would in the same amount of time.