How frequently should you change your assignment for career progression in the Army?

The first few years of your career, generally until you're a SSG, you'll be doing the same thing. This is to develop expertise in your field and your leadership abilities. Once you become a senior SGT or SSG, you will want to move to a broadening assignment like Drill, Recruiter, or Instructor. These are broadening assignments, but they're also called Force Generating assignments. An ideal Army career is moving between Operational and Force Generating assignments. This will give you a bigger picture of how the Army as a whole works and how the various systems within it work and intertwine with each other. For example, can you explain to a Soldier how they will come on assignment in the marketplace? Or how senior promotions work, how to process a compassionate reassignment, or how and when to contact an ombudsman when their family isn't getting the necessary medical treatment?

Once you've hit a point where you have become a Subject Matter Expert in your craft you can start looking towards progression outside your MOS. That means military schools, technical certs, civilian Ed, and job positions.

Military ed- Air Assault, Battle Staff, Master Fitness, SARC
Technical certs - I&P, Lean Six Sigma
Civilian Ed - start working on your degree immediately, it is 100% the separator for promotion at Senior ranks
Job positions - you can't really affect where you are right now until it's time to move, but there are other positions you can move to within your organization, or additional duties you can take that will increase your exposure to your unit and Commander. Training room NCO is a good position for a young NCO to learn about Army processes and systems. Other jobs are CDR/CSM driver or S3 shop. Additional duties that can help you network are P3T, barracks manager, BOSS rep

Well, if you're wanting to get away from the ops side of the house and do broadening assignments/tasks, there is always recruiting and drill sergeant. There is your MOS schoolhouse instructor. I'd recommend SGL for BLC but I think you need to be a SSG for that. There are many different types of broadening tasks you could do.

One of your comments suggested that you would not be interested in crossing over to the officer side. One piece of advice that I have learned over the years: never close the door on anything.

Officers have substantially more earning potential and have numerous “broadening” opportunities available—much more than NCOs. I’ve tried to cross over twice and both times my packet was discarded by the board because of my age. If I had tried earlier in my career things could have been different.

Don’t close the door to your opportunities. You may not see yourself as an officer now, but after you cross over and learn a few things—with help from others, of course, you might look back and say the opposite: “I couldn’t see myself retiring as enlisted.”