It is different for everyone, no reason to change your view! 21 days is a good marker to have. Even if it doesn't feel like the action is set in stone after 21 days, it just proves that you can keep up with it and you're well on your way to a lifelong change. Plus, the habit itself plays a part. Making a habit of drinking only water at lunch, for example, is easier than beginning a daily work out routine, so that can effect the curve of getting used to it.

I only made that comment because I knew about this topic from my college class "Changes and Choices". I wasn't trying to ruin morale by any means!