Re: New sax player: is this normal?
I realize that you posted this 2 years ago, but to help this forum thrive by showing activity, I will provide an answer.
Your embouchre is basically tiring out. It takes time to build its strength, similar to a body builder starting off with lighter weights and lower repetitions.
You build strength in your mouth by practicing regular. If you are tempted to bite down because your mouth is tired, then take a break.
Similar to body building, your mouth will need sufficient rest. As you build strength in those muscles, your playing duration will increase.
For example, when I was practicing 4 hours a day, my playing stamina increased to being able to perform continuously with short breaks between songs, 4 hours without tiring.
Similar to body building, you want to avoid injuring yourself, because similar to body building, an injury takes time to heal. You can set back your progress if you are not careful.
Also if you tire out quickly, perhaps try a softer reed. You want one that allows you to play through the dynamic range of the instrument, the low notes as well as the high. As your muscles build strength, then you can try slightly harder reeds. It is similar to a body builder increase their workout weights.