Three hours will allow you to cover all aspects of playing, but 45-60 minutes is enough for one sitting. The quality of the practice is more important than the length of time it takes.
Practice has several basic objectives: sound, slurring, tonguing (single, double, triple), phrasing. The Arban book [Arban's Complete Conservatory Method for Trumpet (Cornet), published by Carl Fischer, 192.] is set up that way.
Try to get as rich and pure a sound as you can -- an "unbrassy" sound, the kind with no metal in it. Louis Armstrong is a good example. His sound is really bright, but not brassy. It has a core that is warm. During the first 15-20 minutes play long tones, soft, from second line G down to low G. For the next 30-45 minutes work with pages 5 and 6 in the Max Schlossberg book [M. Baron, publisher], varying the dynamics and the tempos. Try to play through every slur, getting an even, round sound on every note, and getting over the breaks in the instrument. Also, exercises 59 and 60 in the Schlossberg book are good to strengthen your lips.
Take a break.
Use the Second Study (page 8) in the Herbert L. Clarke Technical Studies [Carl Fischer]. Work on velocity, with a metronome, in major and minor keys. Slur some, tongue some, and double tongue some. Also work on the "kah" syllable. Go straight up the scale, starting with the middle C (exercise 32). In the Arban book there is a series of exercises to work on your single tongue attack. Number 19 on 28 is especially good. Try to get a nice round attack with some "pop" in it.
Then you can open an etude book. Theo Charlier: Etudes Transcendantes [Leduc] is good for advanced players, or the Arban book for others. Do some double/triple tonguing exercises. That's another hour on tonguing.
Take a break.
Now deal a little more with slurring, but not too much; you don't want to kill yourself. Work out of a book like Advanced Lip Flexibilities [Charles Colin, author and publisher]. Then do some phrasing exercises out of the Arban book.
Finally, play some characteristic studies from Arban, or etudes from Charlier or Schlossberg. When you play these etudes, or any exercise, always go straight through without a stop the first time. Then go back and practice the places you had difficulty. Play everything -- no matter how trivial or trite it might be -- with dynamics and sound and musical expression.