That actually makes a lot of since and its probably my fault as I enjoy training and I am still used t Smokey he will go for an hour enthusiastically until he gets it. He works for the praise not food or toys. I thought I was slowing down with him. Didn't realize they should be that short.
I think that learning to quit when you're ahead can actually be one of the hardest things to get as a trainer, so don't feel bad. It's really easy to do, because we look at training differently than the dog does.
Your goal should be to end every training session on a good note, and ideally with the dog wanting more. So that means that if Chief is starting to really get it and seems interested and excited, stop! You won't want to, because you're thinking, "Woohoo, he's finally getting it! Look how much he's enjoying it!" And he is...but then he'll lose focus or get overwhelmed and you wind up quitting when he's not really that into it.
If you quit when he's excited, then the next training session is going to make him think of that. He'll approach it from a happier standpoint--"Oh, I remember this! That was so much fun!"
If you quit when he's not excited but not cowering yet, at the next session he'll be like, "Oh, this. Okay." And he'll get into it, but you're starting from a lower level of interest so you have further to go to get him really engaged.
And if you quit when he's cowering (which at that point is the only option, of course), the next session will start with him unsure and scared, and you're basically starting over each time.
So that's the mindset you need to work on--end every session at a point that is going to set you up to start the next session in a good place! And remember, we're talking very short periods of time here right now.
If you do make a mistake and go too long, don't get upset about it either. Just try to learn from your mistake and remember that you have plenty of time to make up for it. Also, pay close attention to his body language when you work with him. I would be surprised if he really melts down out of nowhere--most likely, he is giving you some subtle cues that he is getting stressed out. They can be really difficult to see, but if you learn to read his body language really well, you can hopefully stop before he becomes so overwhelmed that he has to cower for a long time.
Also, I love clickers for fearful dogs like Chief. It mostly just takes practice to get the timing right.