I see many people have answered that mental practice doesn't hold a candle to actual physical practice and I want to make sure the value of mental practice isn't dismissed.
When you are sight reading something for the first time you take care to observe details of the score: the clef, time signature, key signature, shape/contour/line, dynamics among many others. While many of us may take this moment for granted, I feel that this demonstrates the value of mental practice. Before you set up at the instrument you have an idea of what you're playing and you won't waste nearly as much time correcting mistakes because you've already caught some of them.
This is a very small example of something that has become second nature because of how we ingrained it as part of our routine when we were first learning how to play. We need to similarly ingrain other skills into our routine such as audiation or score study. Audiating an entire piece will not only help you catch errors on an individual piece, but will also develop your aural skills so that audiating becomes more natural and you are able to catch errors on pieces you've never heard.
Score study is also related and a completely necessary part of performance preparation. Knowing what another instrument or your accompanist is doing where is only one of the many benefits you'll enjoy putting work into this area. Being able to identify why a composer does something will allow you to bring true musicianship to a certain phrase.
That being said, I've only briefly mentioned a few very simple and widely practiced aspects of mental practice, but understanding their benefit is essential to answering your question. The goal of mental practice is to facilitate your musicianship and to maximize the efficiency of your rehearsal. When you play an instrument (especially a brass instrument), you need to make sure that you don't waste a single minute of time with your instrument. I should also mention that the goal of physical practice is to facilitate your musicianship so that you can perform without technical issues inhibiting your ability to play the piece you're working on.
In short, both of these types of practice are essential to be a true performer. You need to spend as much time as it takes doing mental practice to maximize your physical practice time and realize the musical interpretation that will make the piece yours. You need to spend as much time as it takes doing physical practice to maintain your present technical facility and to be able to execute the performance. Your teacher can probably access your level of skill to determine how long you need to spend on physical practice to maintain your skills and you may want to ask him what he expects from you.