I tend to work in 5-6 minute blocks with walk breaks in between. With a young horse, the first block might just be some walking and trotting, changing direction to warm up. Then the next one could add in some lateral work and more transitions, and the third one adds some canter, and then you’re done and cool down. So that is like 15-18 minutes of “real work” with about an equal amount of walking on a free rein interspersed. The walk breaks can be used to see how he is breathing, what his general expression is like. The BO at my stable is an FEI rider and has been training for several decades, and always says she thinks they get mentally tired before they get physically tired, and I think that’s right. The walk breaks can allow them to regroup and settle mentally just as much as catching their breath and giving the muscles a break.
ETA: If I’m introducing something new of course I don’t drill on it for six minutes, I’ll just slip it in with easier work, or touch on it for 1-2 minutes and then give a walk break.
Of course, working muscles in new ways results in soreness, which is totally normal. Learning some basic bodywork like Masterson is really useful to just check in with your horse and how they’re feeling to assess when the training might require some adjustments or they might just need an extra day off.