Sculptris virtually takes NO time to learn because there is NOTHING to learn.
The time you invest when working with Sculptris actually goes straight into your creative process, like working with clay.
If you get good results in Sculptris, it's because you developed a feeling for proportions and how things should look which is directly usable in any other program (after you learned how to use that other program).
So doing stuff in Sculptris is no waste of time and on top of it you can always export to OBJ and use those objects in any other 3D software later.
Just have fun without overthinking all this shit too much and get to it. Create stuff. Get better at it. Create nice things.
JUST DO IT, DAMMIT.