How long will it take for humans to fully adapt to walking straight?

When people decide to take better care of their bodies by eating right, having good posture, and stretching/exercising daily.

There is a concept called nature vs nurture. Nature is inherited whilst nurture is acquired through environment through someone's lifetime. Disorders and disorientation in anatomy/physiology cause the back pain you mention, which is in turn determined by daily habitua.

Relatively to what you can be doing during your lifetime, nature is merely a small percentage of it when it comes to issues such as you mentioned.

It will never happen, unless the people who are better at walking straight have more children than the people who aren't.