I’d say around 5 or 6 pm, or after you’ve eaten dinner.

Evening officially begins at 6pm, I generally avoid “Good night” until I know the person is going in for the night and/or going to bed.

Huh… I never really thought about it. I think I use “evening” after about 4, and then actually switch to “night” around 7-ish.

@MacBean Are you in the US South?

I start with “good afternoon” around 1 and then “good night” at around 5. I think it’s easier this way because if you get in the habit of saying them all you wouldn’t say it’s afternoon when it’s 5 pm….

I would not say either of them. I would just be always saying: “Take care, now, y’all !

That efficiently covers all situations and time scales with a little charm of the old South.

@grumpyfish: Nope. The north. I’m a NYer.

When the sun gets low enough that the light starts to change I would switch to evening, I wouldn’t good night except when it was dark, and probably only if I was planning to go to bed soon.

In Italy there’s a whole standard for when you go from Buon Giorno to Buona Sera to Buona Note (I’m sure I spelled all that wrong, I learned all my Italian by ear) but I can’t remember what it is.

I work in a doctor’s office, and I usually start saying “have a good evening” to exiting patients around 3 or 4 PM. It seems pretty arbitrary, but you might consider your audience. (Most of the patients in our practice are geriatrics, so dinnertime might be just around the corner by late afternoon.)

Then again, on Monday I absentmindedly told a patient to have a good weekend, so maybe I could learn something from these answers, too.

As soon as it looks like it’s getting dark out. That can be as early as 4:30pm near the Winter Solstice in the Northern Hemisphere.

at 18:00

Good question, I’ve always thought about this myself when I was a cashier. Here’s my rules:

“Have a good day”: Opening – Noon (Or until 5 and omit afternoon and evening)
“Have a good afternoon”: Noon – 4ish
“Have a good evening”: 4ish – 6ish
“Have a good night”: 6ish – Close

I hated when I’d mess up and say “Good morning” after dark (or some variation)... awkward!

Both expressions make the enamel on my teeth wrinkle. They are vapid and meaningless, particularly coming from strangers. The check-out people at the super market have been trained to say this. Ugh.

Say: Hi, Hello, Bye, or Thank-you.

I usually just go with “Thanks for coming in” to avoid the time issue.

@gailcalled You’re right, such things do sound vapid because I’ve been trained to say them. But I always do feel genuinely thankful they came in – otherwise our schedule is all messed up. And sometimes they’re people I really like. I work in a doctor’s office, too.

when it’s dark?

I steer clear of the whole mess. When I worked overnight at a truck stop, I usually said “Have a good one.”
I always had more of a problem going from good night to good morning. If someone came in at 4:00 a.m. and had just woken up, they would find it weird if I said “Have a good night.” But if some kids came in at the same time after partying all night… you see where I’m going.
I had a coworker who would greet people according to his body clock. We worked overnight together and he would say “Good morning” when people came in, but he was charismatic and could get away with it.

@gailcalled: What if you genuinely hope the person has a nice day/afternoon/evening/night?

@MacBean if it is genuine, it usually sounds genuine, but many times I hear “You won’t be missed”

Not me, I simply say “Have a good one” because the phrase is essentially meaningless anyway.

Precisely 6 pm since it is sort of the universal sun-down signifying the coming of night, even in the sun isn’t down at your location, 6 pm is rather universal when determining the borderline between day and night.