When I started training I thought I would look like Arnold Schwarzenegger in six weeks. After that I planned to stop training and just enjoy having a perfect body. Of course, as the six weeks progressed I realized that it wasn't going to happen, so I extended my training period and went in search of more "modern" training routines and bodybuilding supplements ...because what I was doing just wasn't working as quickly as I thought it should. This, combined with big promises made by the bodybuilding magazines and supplement salespeople, is how most beginners get drawn into their eventual roles as dollar donors for the multi-billion dollar supplement industry.
Pipe dreams and promises aside, if you're a typical beginner you can realistically expect to build about ten to twenty pounds of muscle in your first year of serious training. Structurally very large men may even get closer to 25 pounds under the right circumstances, while structurally very small men may max out at under 10. This is partly common sense, for one would not expect a man who's 5'6" tall with 6.5" wrists to be able to build as much muscle as a man who's 6'2" tall with 8" wrists. Of course, the commercial bodybuilding magazines and websites usually won't tell you this, but I wouldn't really expect them to. In reality, 90% of them don't seem to actually know enough about training and nutrition to even make a qualified guess anyway (sad but true).
Muscle gain, with proper training, nutrition and rest, follows an exponential decay rate that can be predicted reasonably accurately by wrist size (which is positively correlated to lean body mass in large population studies). A trainee's expected amount of muscle gain during a particular year of bodybuilding training can be approximated by the following equation:
muscle gain in one year = 0.3 × wrist2 × 0.5(no. of years training - 1)
Where "muscle gain" is expressed in pounds and "wrist" is the circumference of the wrist in inches. So, a man with 6.5" wrists could be expected to gain about 0.3×6.52×0.5(1-1) = 0.3×42.25×1 = 12.7 pounds of muscle in his first year of serious training. In his second year of training he could expect to gain another 0.3×6.52×0.5(2-1) = 6.3 pounds of muscle. A man with 8.0" wrists may gain about 0.3×8.02×0.5(1-1) = 19.2 pounds of muscle in his first year of serious training. He could gain another 0.3×8.02×0.5(2-1) = 9.6 pounds the following year.
Do keep in mind that these are merely rough approximations that do not consider the trainee's height, exact physical makeup (which would be practically impossible) or exact training, nutrition and rest habits but, for the majority of trainees, this method will provide a sufficiently accurate estimate - though there are very gifted people who may be able to slightly exceed these predictions (by a few pounds at most). There's also the case where a person has trained for awhile, but has done so sporadically or poorly. In such cases, it's more difficult to make predictions and the trainee has to examine his past to determine how much muscle he's added so far and how far he is from his ultimate potential. The closer a person is to their potential the harder the gains are to come by and, likewise, the further a person is from their potential the more likely it is that they can yet make relatively fast "beginner like" gains.
Also, yearly gains will not be spread out evenly over the course of the year. Typically, after an initial break-in period when a person learns to do the lifts properly, they make their fastest gains early in their training "careers" and muscle gain rate will slow down every month and week thereafter, although it won't be strictly linear - there will be periods of ups and downs. So if you're a beginner you'll notice some quick gains at first but that will slow down to the point where gains are generally not noticeable from week to week. If you're an intermediate you may also make a ten pound gain in a year (particularly if you're a large-structured person or haven't trained properly in the past), but it will require more dedication and persistence. If you're advanced, gains will come very slowly indeed. After 10 years of proper bodybuilding training you'll pretty much have maxed out your drug-free genetic potential in terms overall muscle mass. After 5 years of proper training you'll be somewhere around the 97% mark. In three years it would be about 88%.
Also be aware that "yo-yo" dieting and overeating can produce sudden, even seemingly miraculous, swings in body weight - both fat and lean body mass - but these changes are rarely permanent and body weight normalizes again after the person returns to their more typical diet patterns. Carb depleting and loading itself can produce fluctuations of 10 pounds of body weight in just a few days. But the vast majority of the weight lost or gained is fluid and labile proteins and the weight change does not represent a true long term change in body composition. At the end of the year, people experiencing such fluctuations will still generally fall within the bounds set by the above equation.
Interestingly, speaking on the subject of an experienced drug-free bodybuilder putting on just 6 pounds of muscle in a year, two of today's top natural bodybuilding champions, who both had almost 20 years of training experience, had this to say,
"Six pounds in a year? That's not natural!" and "I haven't put on 6 pounds of muscle in the last 10 years, let alone a year."