A say "Dobry wieczór" when it's getting dark:)

I think that 18-19

In Russian the phrase is relevant from 18:00 to 00:00, which is the same in either English or Polish, I think.

There are no clear boundaries on at what time you should start saying "Dobry wieczór" ;) I guess you're right that six is a bit early, but perhaps seven, eight..