This is pretty much a
rsart - Rick Stirling, games artist » Blog Archive » How many polygons in a piece of string? question,
but this (for characters):
rsart - Rick Stirling, games artist » Blog Archive » Yes, but how many polygons?
and this (for environments):
Rules & Regs - Unearthly
should give you some idea. There is no "correct" answer, it varies upon game genre and engine and hardware capability.