I think the biggest lesson here is get in tune with your body and listen to it. It will tell you when it needs time off. Overtraining is an issue to watch for but at the same time, it depends on what it is. I have 20lb dumbbells sitting around. When I’m watching hockey, I’ll grap them and do some different arm excercises sporadically. I don’t have an “arms day”. I would say I do different curl excercises 3-5 days a week these days. If I pick them up and do one set and by the end of the set, it feels like it would normally feel have after 3 or 4 sets, I just stop and put them away for the day and go back to it the next day. Some weeks are different than others. In the end, I am gaining more this way than when I was doing it just once a week. When it comes to my running/cardio, I can do it every day without a problem but I always take one day off a week just to reset my system. When your body gets used to it every day, it plans for it, which is ok if you are just trying to maintain what you have or perhaps lose weight at a very slow pace, but if you want to lose weight a little faster, it can be a benefit to have somewhat of a “cheat day” where you don’t work out and eat more liberally just to let your body know that the routine isn’t a part of your everyday life and it does not have to keep storing more and more calories each day to prepare for it. Also, and again, you should listen to your body, but when it comes to tendons and cartilage and stuff like that, I don’t know that you can “feel” that kind of wear and tear necessary. In the last year of working out regularly, I’ve had a slight twinge once or twice, took 2-3 days off and everything was fine. But that was more towards the beginning of my fitness journey. Now I have built up the leg muscles, especially around my knees and that hasn’t happened in a long time, thankfully. Still, just thinking about the principle of elasticity, it would only stand to reason that taking a day or 2 off would allow the ligaments to snap back. It’s obviously going to be different for everybody. If it’s telling you it wants to go, let it go, if it’s telling you it’s tired or worn, take a day off. This also allows you to further break the routine up and keep it guessing, which is a good thing.