Lvl 90 to Garrison Setup takes 1 hour long: Warlords of Draenor (Alts)
If you are wondering how longit would take questing your alt from level 90 all the way to the Garrison setup, it will take about 1 hour. Remember to skip cutscenes!
From the dark portal to the garrison generation will take you about 60 minutes or 1 hour.
If you just want resources, you can stop the instant you finish the lumber and cull quests. Barracks and Olin as first follower will take another 5-10 minutes. Neither of them particularly affects garrison resources, although you will get to Olin at some point for the follower garrison mission shuffle churn.
The second follower will take about 4 hours realtime for Olin to complete, so you can log out and go back onto your main.
This one hour can be pretty useful if you want to set up the chain of garrison resources!