You are right, at some point, it does become overkill (or ham, depending on what you're doing). The time really depends on what you're brining, what you're using to brine with, and what tast you're looking for.
This is a table we use for general purposes (which I think we stole from Smokin
):
Brining Times SmokinOkie also has an excellent guide:
SmokinOkie's Brining 101