If the question is what sort of change can be expected rather than what you should do:
I’ll use myself as an example, because results I’ve seen in many others have been similar as well, so I’m a reasonable example and I know exactly what has happened long term.
I didn’t start weight training till 30. I don’t think I gained any significant muscle and no height between 18 and then; I simply got a bit fatter. At 18 I was 147 and reasonably lean, say 8% as a guess? So if true, 135 lb LBM. At 30 I was 155 with the extra 8 lb or so just being fat.
My first three years of training, my strength went up by a large percentage, say doubled, but mass gain was not good. This was because I followed Ellington Darden-style HIT to the letter in this period. No disrespect to Darden: he’s a good author and has helped a lot of people and if what he says is taken as being among the things one should consider, there’s a lot to learn from him. Just don’t make it 100% of a program, on personal experience. I ended this phase at about 158 at 6% bf by underwater weighing or 8% by skinfold, with in this case the skinfold being more accurate I’d say. So, 145 lb LBM. Disappointing mass gains indeed. However, very substantial improvement in appearance anyway, and double the strength at least.
I then threw off the HIT blinders and trained more conventionally. By the 6 year point I was at about 174 lb, again at the 8% bf reference, so about 160 lb LBM. I felt like I was now at a point where further natural gains would have been quite slow, a couple of pounds per year perhaps for a while. I felt I had my training genuinely decent: training wasn’t the problem nor was nutrition.
I underwent some high stress as well as being sick for about 3 weeks and both lost muscle and gained some fat, and looked like crap, yet wanted to have better muscle size for a given date. Was low 180’s. Did a 2 week cycle and was 205 on the date with considerably more muscle. Subtracting out added muscle glycogen and glycogen associated water weight and imagining I might have gained 2 lb fat during those weeks from big eating, still 10-12 lb of muscle put on.
With subsequent cycles I got to where 205 at mid 6% bodyfat was easy and normal as my off-cycle condition, so about 192 lb LBM. At that condition, up about 32 lb LBM from natural best, and about 70 lb LBM from starting point.
Biggest I was was 224 at 10-12% bf (not measured), so about 197-202 lb LBM.
This is all with what I consider conservative use (except some would say that gram/week or moderately gram-plus per week isn’t conservative.)
While it can illustrate gains, hopefully it will also illustrate waiting until having the act together with training and nutrition.
A huge problem with starting steroid use before having the training and nutrition genuinely where they ought to be is that steroid use masks all kinds of errors. And a dependency gets set up where, basically, it seems hopeless to train without steroids.
I would get the training and nutrition more solid, to where you can be more confident of the value of the other 34-35 weeks per year (if using steroids 1/3 of the weeks of the year.) If using steroids too soon, those other weeks of the year tend to suck and continue to suck.