Do you necessarily have to outgrow it? I play a Lafleur tenor, and I have been blaming it for the last few months for my bum notes, and inability to get the low notes, bottom C down. Last week I tried a mouthpiece I bought some months ago, a Vandoren Java. When I first tried it I could only play it for about 10 minutes before becoming puffed out, so put it back in the drawer. (It has a gap of .110")
Could that quote be altered to:-The flippant answer is "when you don't have to ask this question". Actually I believe there is some truth in this. Your level of competency will usually bring with it the ability to evaluate whether your equipment is holding you back.
having said that, theses days there is very little difference between a student and pro horn. I use what many people would describe as a student soprano for pro work.
... So it may be that a combination of some questioning of what you currently play together with some trialling of other instruments should provide a ready answer to your question. If not, you are probably not at a sufficient standard play wise and/or listening wise to need an update.