I don't generally hear anyone speaking English use the term "afternoons."
It is true that when something is routine you can use the singular, afternoon. Basically the each/every component that allows you to make it singular is implied. If you asked How do you spend your afternoons? Then it needs to be plural. It is not wrong in duo's sentence, although I think in the afternoon sounds better.
What do you do in the afternoon is a better translation (i.e. it best conveys the meaning), and should be accepted. Spanish doesn't use a singular time of day to refer to a routine, but English does, so that should be translated.
As a southern US english speaker I do not use or hear afternoons in routine conversations.
Im from Australia and it's pretty common to hear, "what do you do most afternoons?" Etc.
Dont know why this is downvoted? He's right its common to hear "What are you doin this arvo?"
"What do you do in the afternoon?" (singular) should be acceptable. In English, this implies "what do you usually do during afternoons."
Your understanding is incorrect. (There is a pleasant vagueness about tardes but afternoon would be appropriate sense.) Where you are wrong is about the earliness of "buenas noches". Unlike English and French this is not meaning "Goodbye, off to bed now" but rather is also said as a greeting at the start of the (late) evening/night.
Since people are discussing aft or eve does the word change based on it being light or dark outside (sun up or sun done), basically or what? Please respond for it would help the majority of us!!!!!!