Guys above are absolutely correct. Most folks that claim they're overtraining aren't eating well, skimping on sleep, or both. It seems so cliche, but progress really is about what you do out of the gym, not just inside the gym.
Back in my 20s, I trained 6 days a week. I put on muscle and got strong really quickly, but then things grinded to a halt. I'll never forget I took off about two months one summer and worked on a ranch in Kansas. We worked hard, lots of tough physical labor, but we also went to be early as there was nothing to do at night. Probably slept 9 hours a night there, and ate like a hoss. I came back so much stronger! And within a week or two, my physique springboarded and I was way more jacked and striated than before that stint at the ranch.
Morale of the story is its case-by-case, but if you aint busting ass in the gym and eatng and sleeping then you wont see gainz.