After looking at the volume that a lot of pros follow and their career... I dont think over training is really something to worry about unless you are going from doing nothing to training like 2+ hours a day. I see no reason why someone who wants to add more volume to their workout cant over time. Or add more intensity. Their is a limit, just it's safe to say unless you are doing some that is severely out of your normal scope of exercise, overtraining will take a LONG time to get to you. Unless you are stressed beyond belief.