I honestly have so much stuff to do, that I could be here for the next 15 years, I would probably, still have a lot to do. 
But this is extremely dependent on how many things you like to do in this game and how much time you have for them.
The more things you like, the more you have to do.
I can safely say, I missed every objective I wanted in this game, due to lack of time.
The less things you like the less you have to do.
I believe if people only like to play one character and one spec, on average of four hours upwards, chances are they are going to run out of content extremely quickly. 
This is my case:
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Trying to get the last two pieces of my leather basic set from Darkshore, Warfronts.
It’s 23 minutes till deadline and I am not going to make it.
Update: Warfronts are over got another cape that I already owned.
Still missing the gloves and the pants.
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After that I still have the elite sets.
Got to complete the Zandalari campaign in four characters (Plate, Mail, Leather and Cloth) in order to unlock the quest that rewards one piece every time. -
Even though my 10 characters average 410+ some of them are wearing 375 Azerite gear, so time to farm LFR for 430 gear.
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TW grants one piece of 445 ILV and I already got a full set of plate thanks to my 2 Warriors and my Paladin.
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Still need to learn all of the specs of the:
Warrior, Paladin, Hunter, Monk, DH, Priest and Mage. -
Still need to level up a DK, Shaman, Druid and Warlock.
Should I level them up in Horde or take the chance and level this four on Alliance side ?
I made a topic in the RP forum, but I guess my question was just too complex to be answered.
Need help creating Alliance characters for SL -
Trying to simplify my tasks at the moment.
Just going by the calendar.
Like TW event, LFR. -
I still need to finish BFA and all other expansions before.
I still have classic to play. 
Cheers.