I've just finished retopologizing some sculpted curly hair. That took me about 3.5 hours to get to the final polycount of around 1700 polys. That's an extreme example since it was retopo'ed from scratch without re-using bits of a base mesh. It also doesn't have a nice regular surface so most of it was done painstakingly…
I'm writing a GPU powered particle system to decimate meshes, hoping to speed up the pipeline as a whole, it be nice to have numbers as justification but not completely necessary. Bit of a curiosity if there is some kind of professional guide lines. I know people like to say when its done; but I'm sure somewhere there is…