Just a guess since I have no idea how subdivided (ie. complex) your model is…
Model at the lowest vertex resolution possible without compromising your shape, then do the boolean and clean-up will take far less time. Once it’s cleaned up, add subdivisions if necessary before exporting to Sculptris.
If you’re unsure what I mean by vertex resolution:
If you can build your model and do the booleans with un-subdivided cubes, that’s the lowest vertex resolution possible for a 3D object. If you need circles, cylinders or any other kind of round shape for part of the object, use eight-vertex circles or cylinders with eight sides.
I often find it’s easier to do booleans by hand rather than letting Blender (or any other 3D software) do it for me. It may take a bit longer, but I have full control over the results.