Steve, I am about to take a short answer and make it rather long.
When I was six years old, my idol was the local trash man. He would pull up in front of our house and grab this large silver trash can off the back of the truck. He would proceed to dump our garbage and the neighbors garbage in that container and then with on hand, flip it up and around onto his back. He would walk back to the truck and dump it into the truck. He was huge ... the biggest man in my town. He did this 5 days per week, 8 hours per day and retired after 30 years on the job, when I was 20 years old. So what does this have to do with your question?
There were probably 20 other men that did the same thing for a living. Some lasted a year and then their bodies could not handle it. Some lasted from 1 to 5 years and a couple more even lasted as long as 10 years. My Idol lasted 30 years.
There are so many different ingredients in the mix that I do not believe there is any one or any three answers that would apply to more than 20% of the population at any given time.
Short Answer: There are no shortage of opinions on this topic, but there may not be a consistent answer to it.
"Today is always the perfect day to train"