Disney advises something around an hour to 1.5 hours, and I FULLY agree with that.
You should be LEAVING the front of AK's gates around 4:00pm. You still have to wait for a bus to show up, then get to the other side of Disney property, and the bus will stop at the Poly, Contemp, GFlo & WL, I'm not sure about the order but I THINK the Poly is the second to last stop.
Especially at closing time of a park (although AK is usually not that busy) you never know how many people will be there, how fast the bus will come, and how fast it will leave.
If you arrive early, either check in early, have a nice lapu lapu at the bar, or wander the grounds, check out the beach & explore for a bit. Always better to be early than late!