If I just do the quests in Zandalar Forever! achievement (I think it’s 6 parts) how long would it take?
Also, do I need both requirements on the same character for the Zandalar allied race unlock? Or can I have 2 characters with each of the achievements unlocked 
Asking for a friend thank you!
I think you need it all on one character; my suggestion simply cause tha amount of hours you will put in to complete it is to just make a new character and level it through BFA… so by the end of it you have a new character and you have the achievement done.
ALSO FOR COMPARSION; I made this character last week; to do Unititng Kultiras/unlocking kultirans. I finished two zones Drustvar and Tigardesound by level 50 that was like 7 hours in; i finished all 3 roughly in 11 hours. (The war campaign;Jaina story and all the zones chapters)
I’m just doing it on my iLvl 130 Shadow Priest. My thoughts are that since it’s such a powerful toon I can just plow through things so fast… I’m already done with Tides of Vengeance but I need still need all of Zandalar Forever!
I had Zandalar Forever! 2 zones done on an alt but the ilvl was cancer (ilvl 60?) and didn’t want to barely survive through the War Campaign so I decided to do it on my main that I xfer’d factions
Ehh you can try it but you have to factor in scaling; I was breezing through content and soloing group required bosses cause of the scaling.
So it wasnt like i was a weak toon; slowly killing the mobs.
TO me personally I could have done it on my level 50 Draenei paladin but I new in the amount of hours I would have put in i could have just leveldd a new toon; so I did and now I have this pandaren monk.
If you check my achievement you will see I started on 11-06 and finished by 11-09
To unlock the trolls, you have to do the whole achievement on a single toon. The Alliance can do theirs on different toons for different parts of the meta, but for us poor Hordies, it has to all be on a single toon.
/moo 
If you just do the main quests on each zone, skipping the optional side ones, then probably a couple hours per zone maybe. When you finish the three campaigns you’ll get a quick questline at the Blood Gate for that achieve then the final scenario for the last one.