Fixing bad posture takes time. The amount of time it takes will depend on your personal history and your daily habits / lifestyle.
Some people see improvements almost immediately. But for most people, it can take a few days to a couple of weeks before they see some signs of improvement. After that, it's a question of how much of the right stuff you're doing versus how much your daily habits drag you into poor posture.
In some cases, it can take a couple of months before you see dramatic improvement. If you've had a hunchback for decades, you should expect it will take at least a few months to make any significant improvement.
Key Takeaways
If you learn to fire the right muscles and reduce the amount of time you spend in bad posture, you can make noticeable progress fairly quickly.
If you have had bad posture for a long time and you continue poor habits/ergonomics, progress will take much longer.