Hello, Nick!
Thanks for asking an important question.
Mock-auditions are invaluable for learning the process one must endure to "compete" in the real world of orchestral auditions. There is certainly more to being a success at an audtition (of any kind) than just learning the excerpts inside and out. Clearly in orchestral auditions, accuracy of rhythm and pitch are paramount, but it would be wise to know the part well enough that you "hear" the orchestra around you when you are playing the part alone. And if by chance the audition committee wishes to hear a passage that was not on the list (sight-reading to some extent), it would be great if you were prepared to do that... accurately and musically. So, to prepare I suggest that you always know the entire symphony (or whatever the work is). Know how the bass part is integral to the expression of the drama. Once you know the music, putting the bass part under your fingers will be easier. Be sure you have decent, modern recordings of all the music you must learn, and have a way to record yourself, so that you can easily, upon replay, hear what the judging committee is going to hear. You then need to be unbiased about critique: whether you were accurate in rhythm, dynamics, pitch, bowings, articulations, and phrasing. There are also "traditional" ways of doing certain excerpts that you should know. Again, listening is the best way to learn these. So, by studying, aurally, the music, you will learn it faster and more complete and should therefore take less time in the practic room. Also, remember that ALL music is made-up of scales and arpeggios and it behoves one to do LOTS of scales and arpeggios practice every day, then apply this knowledge to your pieces.
5-6 hours a day is enough for most humans. HOW you use that time is the important variable. A very methodical, careful and concentrated practice results in the fastest learning. Do not simply repeat everything you already know how to do (like C major vs F# major). One must challenge the barriers (whatever they are) to progress and excell. And if you do something you do not want, acknowledge it, but don't do it again. Repeat what you DO want. Remember that you are adding new ways of doing things, not eliminating anything, only putting those things that you do not want right now, in a less-accessible place in your brain.
Finally, there is no right or wrong way to practice, but you must find what is the most efficient for you, and it CHANGES as you grow. When playing for people, there are always expectations on the minds of both the listener (audition cmt. etc.) and you. How you let those expectations affect you will determine your success.
Best of luck!!
PN
Mock Auditions 
