It might stop working eventually but you won’t know unless you try and if 12.5mg has stopped having a strong effect you need to move up.
Even if 15mg only helps for a while that is still longer to lose weight until you no longer feel suppression.
I had a similar experience as each dose eventually became tolerated to the degree that I didn’t have the initial suppression and so moved up over a year to 15mg.
At the moment 15mg is still giving me suppression.
That said, I have never had the enormous effect some people report though and although mounjaro has been a definite help I have always been able to overeat enough to not lose any weight and have still needed to make a determined effort not to do so.
You might as well just move up to 15mg because what exactly is the downside?
Slightly more expensive but that’s all and there is no point paying for 12.5 mg if it isn’t working and a higher dose is available.
I do think that in general, not aiming this at you OP, we need to remember that mounjaro is said to reduce a percentage of bodyweight and does not necessarily mean that those of us with the higher body weights will therefore lose all our unwanted weight.