One thing I’ll mention is that we get really good at what we do a lot of. That’s why it’s really important to pull back if you start making repeated mistakes. Musicians in general tend to be hard working and a little stubborn. It’s so easy to mess up a phrase and think “Oh I’ll just try it again and get it next time”. Well, if that happens over and over, you are actually repeating the act of screwing up, again and again. We don’t want that!
We’re all different in our thresholds, so just because Steve Vai could practice x hours per day (and stay focused enough to make all x of those hours GOOD practice) does not mean that we all can do that. If your threshold of focus is shorter, make the most of it! If you get to a point where you simply can’t play without being sloppy at some point in your practice, you’re better off putting the guitar down and coming back fresh when you’re more accurate. 15 minutes of good practice, time after time will translate to much better playing over the months and years. Just make sure the practice is good! Obviously you’ll progress faster if you can log more sessions of this type of playing in a compressed timeline…but you’ll still progress!
Lastly, maybe there is something you are doing wrong, technique-wise that you are able to ‘muscle through’ for a short period of time that is simply not possible to sustain once fatigue sets in. That’s why I LOVE CtC!!! It made me aware of the pitfalls that come with trying to play complex, fast passages. My problem over the years was not knowing this, and being too macho and ‘hardworking’ to accept the slop and think I needed to just keep going. That equated to literally hundreds of hours of trying to muscle through what was actually very slight string hopping. If I could get those years back…man! I’ve only had my membership for ~1 month and I’m already playing things with total control at speeds where it used to just all fall apart. Just being able to pinpoint the optimal escape direction for the string changes (or a swipe!!!) and adjust accordingly, is all my playing needed to get to the next level.