Honestly it can totally ruin your progress and I'm not kidding here.
The sad thing is that I am great when I train clients when it comes to not having them do too much work BUT I'm really bad with myself.
The only thing that allows me to get the right amount of work is training with my friend Nick. I design the program for him and I train with him. That way I'm more objective.
But here are some of my beliefs and observations:
If you look at the world's best powerlifters they tend to train hard 4 days a week, sometimes 5 but in that case the 5th day is normally an easier workout. High level strongman also tend to train 4 days a week.
I personally know several pro and high level amateur bodybuilders, heck I've trained many myself. And honestly except for some rare exceptions they tend to train 4 or 5 days a week.
Most athletes (hockey, football, etc.) tend to have 4 weekly training sessions.
Charles Poliquin who (like him or hate him) is a pioneer in the field of strength training uses a 3 days split with the following pattern: 2 on/1 off/1 on/1 off/repeat ...so training 3 days out of 5 (or 6 out of 10).
My most successful clients/athletes train 4 or 5 days a week.
Now there are some exceptions: elite Olympic lifters tend to train up to 6 days a week. But many of these days are low in demand/stress level and are mostly technique sessions.
I have also put out programs based on a high frequency and lower stress per session. So it can be done.
1) STIMULUS ADDICTS HAVE A HARD TIME LIMITING HOW HARD THEY GO IN ONE SESSION. Personally if I plan on doing an ''easy'' workout I will end up doing too much or going too hard. Those of us who like to go as hard as possible, who tend to do too much, cannot use a 6-7 days a week program ... for it to work you need to limit the amount and intensity of work you do in each session, and stimulus addicts can't do that.
2) If individuals genetically built for strength and designed to tolerate hard physical training (and maybe using drugs) can only have 4 or 5 hard sessions per week why would average Joes be able to handle more training than that?
Stimulus addicts have this reflex: take a program by a high level lifter (for example Westside Barbell training) and ADD MORE WORK TO IT. How dumb is that? You are likely a lot less efficient at recovering form intense physical work then they are, so doing more work is the last thing you should do.
3) Do not pride yourself in how much work you do. In reality it doesn't matter. The ONLY thing that matters is how much progress you are getting. If someone is progressing faster than you BUT is doing a lot less training than you are doing HE is doing a better job than you, PERIOD! There are no points for how much work you do, only points for the finish product.
4) I personally find that if I train as hard as I want in one session, I can only train 2 days in a row. If I do 3 hard days in a row I will show signs of a catabolic state (flat muscles, retaining water, mood swings, lack of focus, high blood pressure) and the 3rd workout is usually bad.
I CAN train 3 days in a row occasionally BUT the first workout of the 3 days need to be an easier session. NOTE: the natural tendency when training 3 days in a row is to put the easier workout on day 3, that is a mistake. After 2 hard sessions you will be on the edge of being in a bad place, any workout done in that state will push you over the edge. Doing the easier workout on the first day will make it much easier to avoid that.
But I would recommend that until you are good at knowing when to back off, you should stick to training 2 days on/1 day off.
5) I do 2 big lifts per workout. That is planned. The assistance work done after that can vary depending on how I'm feeling but I try to avoid doing more than 2 assistance exercises, sometimes 3 but rarely. And when it doubt, do less, not more.