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The best time to change is when you have a set goal that cannot be reached with your present equipment.

There are a kazillion threads here about mouthpieces. They all end up the same: people that recommend what they themselves play and people that urge only changing when a benefit is clear and predictable.

I think it is easy to say that a Bach 7C is a good standard mouthpiece to start with. Depending on your breathing, your chops, what you play, how much you practice, a bigger or smaller mouthpiece could perhaps offer a better compromise. That is something I leave to your trumpet teacher.
A bigger mouthpiece can give you a bigger, darker sound but will require more effort to play especially in the high register. A smaller mouthpiece can give you a brighter sound and an easier high range but you need better control of your chops to keep them from swelling and filling up the mouthpiece, cutting off your tone.
There are almost as many brands/sizes of trumpet mouthpieces as players and the search for the optimum mouthpiece has ruined many.
This is why my opinion is:
The best time to change is when you have a set goal that cannot be reached with your present equipment. That is something that can be objectively answered.

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Whenever I feel blue, I start breathing again.