Retopology itself can be done in different programs like ZBrush, Maya, Blender, and others. During the construction of the low poly mesh, create an edge flow direction along the anatomy lines or clothing design lines. The joints require extra attention to look good when it's getting rigged and animated. Do not forget about the technical specifications and the limit of polygons.
When all lowpoly meshes are ready, move on to UV unfolding and layout, after which I bake normals, occlusions, id- maps, etc. All of that is required for texturing. It can be baked in x-normal, Marmoset Toolbag, Substance Painter, or Blender.
For me, it can take from 16 to 80 work hours to complete depending on character complexity (amount of attaches, costumes, straps, and things).
Evgeny Pyrch