I have a 14 yo downhill-built app that has been a trail horse his entire life until 9 months ago when I got him. I trail ride, fox hunt, and have been teaching him basic dressage -- to go straight, stay relaxed, to bend, move away from my leg etc... He's just now able to stay balanced, forward and carry himself. It's hard for him. How long can I reasonably allow him to work before taking a walk break to stretch?
He's fairly fit - we hunted through the fall, but these are all new muscles he's developing and learning to use. I don't want to make him sore, but I also don't want to let him off the hook.
Ditto maintaining the quality. I'd also add that if he really 'gets' something that you've been working on, give him a break right away and then come back to something different - don't drill.
Sometimes it is best to end an entire ride after 15 minutes if you've had a great breakthrough; I've had lessons that were only 30 minutes because of a great a-ha moment and we wanted to convey the message that QUALITY work is what we're looking for.
You are at this point laying the foundation for future work. You are right, it IS hard for them - your horse is now using different muscles than he's used his whole life, and it takes time to build the endurance for sustained effort.
When I first got my mustang, I'd ride him for 15-20 minutes, gradually building up to 45 minutes-1 hour (which includes a nice walk warm-up and cool-down). Sounds like you've got a wonderful cross-training system for your horse, which is great!
I only have one lesson a week - afterward my horse is really tired so he gets the next day off then the day after that is a trail ride. I don't want him to sour to the work since he's really not built for it and it is important to me that he be both physically sound and mentally happy.