buenos tardes/noches/ - what time?
What time of day is it ok to use "buenos tardes" - after midday, or later?
And "buenos noches?" what time again - and is it OK to use "buenos noches "for "Hello" at night time (because in English "Goodnight" means only "Goodbye" at night time.)
Welcome to the forum.
I always taught my students that some of its usage depends on level of darkness. If the sun sets at 4:30 PM, you might say "Buenas noches" at 7 PM, whereas if the sun isn't setting until 9 PM, you would say "Buenas tardes" at 7 PM.
And - yes - it is common to say "Hola, buenas noches" as a greeting if it is nighttime.
Just a grammatical note" both "noches" and "tardes" are feminine, so you will use "buenas" for both.