I thought for sure this was going to take hours so I put it off a couple of days. I was wrong. Oh so wrong. I sprayed the headliner with Goo Gone, let it sit for about 2-3 minutes, and then lightly scrubbed it with my brush. Pretty sure the whole process took about 10 minutes.
Now I just have to figure out what I want to do with the headliner. Do I want to put new speakers in it? Do I want to paint it with bedliner instead of replacing the fabric that will inevitably fail. Decisions, decisions!