In terms of actual gains, depending on the factors listed above, maybe 12-15 lbs. of muscle, 15 lbs. being on the high end. This assumes you are natural, by the way. Drugs are a personal choice, but my experience is as a natural bodybuilder, so I'm not going to pretend to know a lot about how steroids affect your gains. Having said that, it makes sense to at least be aware of what they are and what they do, purely from a knowledge standpoint. There are enough knowledge resources available on line that you can learn about them if you choose to pursue that option.
The use of cardio exercise to get into shape was surprisingly nearly non-existent in the time period from the 1960’s to the 1980’s. As mentioned before, gyms were well equipped with barbells, dumbbells and heavy, basic machines. Cardiovascular equipment such as treadmills, stair masters and elliptical machines were not developed yet. Some gyms would have a couple stationary bikes included in the establishment but they were not nearly as comfortable or as advanced as the equipment that frequents health clubs today.
In my business, (I work in a retail vitamin store), I get customers all the time who come in looking for the magic supplement that will pack on pounds of muscle overnight. When I begin to question their training, eating, etc., I discover they've been training maybe 3-4 weeks; they train every day (sometimes I get a guy who says twice a day), they have no clue about protein intake, calorie intake, recovery, and on top of all that, the routine they use is anything but logical for their experience level. This extremely common!
After session 12, consider whether you need to increase the weight for any particular exercise. If you can comfortably do more than the RM of 12 exercises, increase the weight by a modest amount, say two pounds or a kilogram for isolation exercise muscles such as triceps and biceps, and 5 pounds or 2.5.kilograms for compound and large muscle group exercises like squats and deadlifts. When using dumbbells, this would apply to each one. Don’t increase the number of sets beyond 3 at this time.