How long can you play before your mouth starts hurting? Playing a musical instrument is very similar to physical exercise.
As with exercise, it is very important that you warm up before you start rehearsal or practice. Start with long tones that are easy to produce. Play long tones that are low, high, and all in between. Play a few scales, slowly.
Once you warm up your mouth, work on your tongue. Do some articulation exercises to get things moving.
The more you do this, the longer you'll be able to play before fatigue sets in. It will still happen, but it won't happen until much later.
