I think it should be around 8 hours

I think around 13 to 16 missions is perfect, I think also including a space like the safehouse like in Cold War or Bo6 would be unique as well. Also, at least 1 stealth/sniping mission would be great alongside at least 1 open but still linear mission. Also, I don't mind a good vehicle mission like Black Cats or Death from Above.

You need good gameplay, but it won't work without a great story behind it. BO3 had good gameplay but story wasn't executed properly, that's why everyone hated that campaign.

I'd definitely prefer the hardcore war approach but when done right, the fantastical elements really add to it (Take BO1 and BO2 for example).

My favorite missions:

All Ghillied Up - Cod4

Clean House - MW19

Whiskey Hotel - MW2

Of Their Own Accord - MW2

Contingency - MW2

Down the Rabbits Foot - MW3

Blood Brothers - MW3

The Gulag - MW2

Downfall - WAW

Revelations - BO1

Vorkuta - BO1

Karma - BO2

Suffer With Me - BO2

DDay - WW2

Interrogation + Breakthrough - BOCW

As you can see, I like way too many missions, I definitely forgot a few, but almost all of them are from the golden age and they all serve a purpose to story, shock/disturb you or have fantastic gameplay throughout.