There are many differing viewpoints as to what practice is necessary outside of a full-time job as a musician; for example how much preparation is needed before a first rehearsal, whether the rehearsals themselves are there to practise in and other viewpoints.
In my personal experience, how I use my practice time is never the same, depending on whether I’ve played the piece before, whether we’re on a heavy rehearsal schedule, and so need to pace myself, or whether there is something special both inside or outside of work, such as a concerto, or a particularly demanding opera such as Wagner’s Tristan, that needs preparation.
This article http://www.bulletproofmusician.com/how-many-hours-a-day-should-you-practice/ is rather apt in the approach to take to practice, and is certainly something that I have experienced; hours of time spent on something where my brain hasn’t been present doesn’t lead to a lot of progress. Yet ten minutes focussed on a particularly difficult couple of bars, if approached in the right way, can yield the breakthrough.
I have also over the years had to reevaluate the way I approach things. Practice techniques that may have worked at college – I’m thinking particularly of a month or two spent with Sevcik shifting exercises – may not work in the same way once in the profession, and I find my approach to things is generally more piece driven, rather than general technique these days. But these things are a learning curve, and certain things work better for some than others.
One of my colleagues asked me how much I practised once and I couldn’t give him a definitive answer. Enough to be able to play well what we were working on sprang to mind, which sometimes is a standard that never quite gets reached because some of the passages in Tristan are definitely a work in progress and I hope only for continual improvement! Enough not to look like a complete idiot at work and avoid embarrassment could be another standard, and that can require more practice for certain pieces rather than others. Enough to satisfy my own musical standards; perhaps a closer answer, but one that is different for everybody, and depending on what you are aiming for, can be a problem in itself if different people have different starting points at first rehearsals.
It’s all a bit of a movable feast, and entirely dependent on the musician’s work ethic, and their understanding of what is expected of them, and how much practice they think that requires.