The Oxford English Dictionary defines 'afternoon' as the period from noon to evening. It then defines 'evening' as the time at which 'dusk' takes place...ie roughly at sunset.
It would seem, therefore, that the start of evening is - strictly speaking - moveable! However, given that the sun sets very late in summer, that would suggest that summer evenings don't even start until about 10 pm in Scotland!
As a rough guide, I'm sure your 6 pm idea is pretty generally acceptable.