I voted more than 20 minutes.....Izzy can go about 45-60 and that's pushing it. She was in puppy training since I got her at 5 mos. Puppy I, II and CGC classes run 1 hour and she is very good during that hour but with all the doggies in class you can tell that about 45 minutes into the class attention span starts to fade fast.
I think what keeps Izzy focused (and most of the dogs in training classes) is that we go from one trick/command to another. We focus on one trick for about 10 minutes and then move on to another. We do take a few minutes of break time between each one.
At home I can go about 20 mminutes from being distracted because she is so worried that her cat peeps are doing something and she's not involved.
Just keep adding a few minutes each training session with special treats and I'm sure Eddie's attention span will expand. Do you use treats as a reward? Or just verbal?