From what I've seen it's right away, but you will not really see it unless you leave the phone sit. It has to sit on a fairly solid table... no bumping or even small jarring, then you can see it in the battery settings.
Mine goes into doze pretty quickly. Like fatboy97 said, you will not be able to actually see it in the graph unless it sits for a bit. This depends on the up-time from last charge of course.
I was wondering if I put it on the table when I begin a class and leave it there for 45 minutes, if it's actually working. You're right, hard to see on the graph over short periods but I guess it is working. Every little bit helps!