3 days a week is considered ideal for beginners, you should see results within a few months if you do it right. I'd recommend the book Starting Strength for detailed info. If you have trouble finding a PDF send me a dm, I think I have it around somewhere.

Three times a week is definitely enough. MWF schedules are the backbone of many workouts. I did 4-day workouts for years but enjoy the three day schedule more.

Do at least a bit of cardio on the off days. Even something as simple as an hour-long walk.

inb4 struggle session over how many days are in a week.

It certainly is, but as you get stronger you might find that the workouts get longer, and splitting them up into more sessions is needed. I did 3 a week for years, and when I could really stick with it (and eat and sleep enough) I saw great progress. Although training 6-7 times a week saw the best progress.

ideally each muscle group twice a week but once a week is enough too

eating and sleeping is just as important if not more than lifting. don't focus too much on the lifting part

Depends on what you do. Big compound lifts 3x a week is enough.

Nutrition is much much more important imo. Lots and lots of protein.

Yeah, 3 days per week is fine if you're doing the big main lifts (e.g. squat, deadlift, bench press, etc). I'd do some kind of cardio on the in-between days. But for actually changing your body composition, diet is the most important factor.

While its not nearly as binary as X times lifted, 3 days is enough to see results and keep in shape. It also depends what you're considering results. Here's a really good video on how nuanced it can get: https://www.youtube.com/watch?v=Cx6kz1fpwgY

(Dr. Mike isn't for everyone but he conveys the information rather well, IMO)

Good news is you can stay lean, or build muscle, or try to do body recomp at 3 days. Just make sure youre hitting the sets per week per muscle group and don't sell yourself short. Shoot to get close to failure!

Protein intake is also important (1g per lb of body weight per day is suggested. That's hard for me so I do more like 0.8g). But make sure you consume healthy sources of protein so you don't get high cholesterol

If you have weight to lose, it can also seriously help show results (but don't develop an eating disorder, it can be a slippery slope)

You will certainly see results with that intensity. I recommend cardio of whatever you enjoy (biking, running, hiking, swimming, etc.) at least once per week to make your fitness well-rounded

When I did exercise regularly (though I was in my early 20's so a huge grain of salt with this) and have a manual labor job, it seemed to keep me decent shape.

I wasn't really trying "achieve" anything other than pass some occupational requirements for being able to do my job though.

I never rest, basically workout 7 times a week. If I let myself take a break I'm afraid that I'll lose my discipline

What do you mean by "seeing results"? If you mean building muscle/gaining definition, then 3 times a week might be slower than lifting more often a week, but the largest obstacle is going to be nutrition/rest, regardless of workout frequency. If you mean improving your lifts by following a program and overall mental/physical health benefits, 3 times a week is plenty and a great start.