Hi

and welcome to the forum, and good-bye to the rest of your life

jk
As you have read, it basically depends on your style of play. If you know what you are doing and can just grab some units to throw a force together maneuver them toward your goal without second-guessing yourself over and over you could manage a turn in probably about half an hour, but who really does that.
Once the game really has gotten underway--mid to late '61 onwards--you should plan with about an hour to an hour and a half. Don't forget, bantering with your opponent and discussing thing--how rules work, how they should really work, why Pocus doesn't just do everything
I want

...

jk, but really

--it could add a few tens of minutes.
I've never gotten a PBEM that far

, but I have
allowed Athena to get into '64 and beyond, just because I didn't want to start another game. Playing the Union you start getting burdened down with a lot of menial tasks. By then running the blockade boxes--keeping your fleets supplied and in repair--is like play a boring game of solitaire.
I find myself spending lots of time managing garrisons too. Once the war has moved away from the northern states, line infantry garrisons are no longer necessary. I start replacing them with militia and shifting those garrison units south into territory where they are needed--you take a lot of small cities in the South and if you don't garrison them they will become havens for raiders and partisans--. Still you need to keep building more and training them up with Halleck and Siegel--and McClellan until he leaves--and that takes time too.
Sometimes--only playing Athena--late in the war I've just grown weary of the time it takes to scout out the enemy--sending single cavs into enemy territory to scout, paying attention to their supply levels, making sure you have then next wave of scouts ready to go when these come back--I just stack them in with the corps and wing it. If I've missed detecting a force readying to sneak past my corps-line, hey! excitement

.
So, it really depends on your own style of play and how much time you're willing to put into managing your forces aka micro-management. Just don't forget to have fun.