If you need to build up to it here's something I learned.
Start with taking 1 minutes takes of the first few minutes. Move up to 3 minute take in whole. Then take a 5 minute take in whole.
If you want to take it longer, you can continue in that way with the next section of dictation. Then try to take the entire section.
If you find that you tire easily, you may have to decrease your speed for long takes until you get use to it.
By now, you should know all the tips, such as: *smooth flowing pen *smooth paper *knowing the theory very well *holding the pen loosely *etc. Once you have mastered all of those "tips" you will be able to take long dictation.
Why? --long speeches (usually by political people) --those long meetings you have to attend and need to start alert enough to take down notes --that professor who finally found something exciting to talk about that you want to write down --the "never-ending" podcast --need all the details of how to do the job to get paid
Keep taking notes in shorthand until you feel confident about taking longer takes. When you do take longer takes, you may find that some of the last part will be missing from your notes, but that's okay.