The upper limit for most trumpet players is the amount of chops they have. Don't practice when your chops are tired. Takes lots of breaks.

The other limit is something I didn't realize notice until I started playing piano seriously. There is a point of mental fatigue where you're just not able to focus enough to be making any meaningful progress. This is really dangerous on trumpet as combining mental and physical fatigue means you start practicing in bad habits based on tired, aimless practice with tired chops.

Maxing out your improvement just has to do with the efficiency with which you approach practice. Most musicians, once they get decent, end up spending way too much of their practice time further polishing what they are already good at. It strokes their ego and gives them that nice little dopamine hit. It feels great to get your C major scale just a liiiitle faster, but it feels awful to slog through scales you're less comfortable with. But what makes you feel good isn't what's necessarily good for you in terms of improvement.

Just make sure your fundamentals are in place so that you're playing physically efficiently, then move on to things that actually need work.

Because of the way our brains work in terms of making connections, 1 hour on a single thing in one day is way less helpful than 6 10 minute sessions spread out over a week. You literally can't just cram it. Even if you didn't have the physical barrier that trumpet presents, it just doesn't work. Your brain needs time to make connections and it will connect whatever you give it. So if you practice badly, that's what it will form into a habit. Breaking those types of habits is brutally difficult.

You're better off spending an hour on several different things, each for short periods. Diversify your daily practice rather than deciding "today is melodic minor scales!"

But set upper limits so you don't just get obsessed with improving things that you start to get good at. It's better to be able to play all of your major scales at 80 than to be able to play on C G and F at 120 while the rest are crap. When it comes to keys, within reason, don't allow yourself to work on one until the rest are at a similar tempo. Same idea goes for pretty much any concept.

Your maximum improvement comes from how many things you can juggle. Just don't juggle so many things that you can't give focused attention to them each individually. Also, if you really start working only on things you suck at rather than stuff you're already decent at, you'll definitely start hitting the wall of mental fatigue. Be ready for that and make sure to stop when that happens, even if your chops still feel good.

So worry less about the amount of time you're practicing and more about the quality of that practice time.