Well, generally almost everyone experiences amazing results up front and they slow down with time. Its just like when you first start training, the first 6 months you make awesome gains in strength and they slow down afterwards.
It is not so hard going from 13" arms to 15", going from 15" to 17" a bit harder, going from 17" to 19" a HUGE difference. The diet starts becoming a pain, and you are fighting somewhat inhuman levels of muscle. Plus mass is not linear. As for strength, it is much easier to go from 100 pound bench to 225, than it is to go from 225 to 350.
If you look at how the pros were say 5 years before their previous physique, even if they are "new" to the pro scene, they were not small at all. It takes a very long time to get where they are, and yes things do slow down with time. Some people with genetics and amazing dicipline can get hustling extremly fast, I have heard of 70 pounds LBM (claimed) in a single years time by a very serious individual. However that person is now dead (abused a good number of recreational drugs).
So anyway the answer is, there is no way to answer really. It depends somewhat on your genetics, and your knowlege, and how far you can push yourself. For me right now my problem is diet. I have occasional days where I go over 6k, but I am already finding it a chore to eat in the 5s every single day. So for those guys, who 360 days out of the year or so eat 6-8k calories to maintain, well hell it takes some doggon work.
So IMO, most peoples gains slow down because it gets difficult to push yourself past those road blocks. I think it has less to do with drugs, than it does dicipline with diet and training. However the tendons in your body have limits that you would be wise to obey. Too often people gain strength quickly, and find injuries. Plus there is always the claim that many types of gear degrade tendon quality, so be carefull and listen to your body.
So really, you just have to see how you do on your own. Keep counting those calories though, the only way to do things right is to treat it like a science. Calories go up bodyweight and mass goes up, calories go down too fast and you dont just drop bodyfat but you drop mass too.
Before I ramble on forever, good luck.