Is there a way to all this quickly and efficiently if there is a very complicated mesh to retopo? How do you decide how much detail to keep?
There are great auto-retopology tools out there now:
DISCLAIMER: These tools are amazing for generating fast *quad-based* retopologies. BUT they're not suitable for specific topological detail that comes with by-hand retopology. Put another way, auto-retopology tools won't be used on main characters in animation films any time soon (if ever). But I can see it being used for secondary / tertiary backaground assets/characters.
So I wouldn't recommend these auto-topology tools for characters like Piero but if you want something quick and dirty, the tools listen above are useful.
ggdhprinter I like Kent's response. If you want a quick and dirty have something do the heavy lifting for you. If you want a masterpiece you are most likely going to have to paint it yourself.
How much detail you keep is really up to you. Fake it where it makes sense and where there are things that seem too big to pass up go ahead and put extra effort in those areas. There is some good content about this in the retopo videos and what types of details to keep and what to let go.