All miniature commissions start with this step as it is essential to get good end results and a good finish/paint adhesion. Whether you are getting a simple miniature painting commission or going with a custom be spoke miniature painting commission service they all need to be prepared in some way or other.
As part of providing a professional painting service I believe this is equally important regardless of the painting or basing tear selected for the miniatures and endeavour to hunt out every mould line and fix any imperfection.
The time taken to do this can vary wildly from material to material sculpt to sculpt and size so it is estimated on a commission by commission basis.
I love prepping and building miniatures as this is where the groundwork is laid for an excellent paint job.