Some people would say not too dark, birds need some light to go about their business during the day. Dark enough while you can still see some outlines of the columns, beams, walls or whatever.
Many "consultants" publicly claims light levels should be less than 1 lux; while some say less than 2 lux. Recently I learned that 10 lux is OK too!
So when you ask different people you get different answers. Well what is correct? May be there is no "one answer" - unfortunately we can't ask the birds! May be it is not such a crucial factor.
To begin with, how bright is 2 lux or 10 lux? So I finally decided to get a lightmeter. It is "good to have" and not a "must have" I think.
At least now I have an instrument to make measurements & obsevations to form my own opinion.