When should I expect the AT&T Galaxy S5 Lollipop update?

No one knows for sure. It's rolling out now in Poland and parts of Europe. The buzz on the street is that it will most likely arrive sometime in the month of January, unless you're a Verizon customer. For some reason, I'm told they will get it a little later.
 
The Sprint variant already has a leaked build that you can flash if you're comfortable with Odin.