For a new shape from scratch it usually takes me about 3-4 weeks to make the first skin. Once I have that first skin it typically takes me from a few hours to a few days to do another scheme using the first one as a template.
This thread is just me updating my already submitted skins to AH 3 format by making the new maps and generally tidying each one up. The first time I update a template this way it takes me a few days, after that an hour or two per skin.