Practice: 15 Minutes is Better than 1 Hour!
If you're focussed on what you are doing!
15 minutes of careful practice with a pre-determined goal is much more beneficial than 1 hour of goofing around, playing a few excerpts, jumping from solo piece to solo piece and occasionally playing your favorite lip flexibility as fast as possible. If that sounds like your practice then it's time get focussed! Developing short focussed practice sessions will benefit you in the long term if you're serious about progressing on the instrument.