For myself, it was immediate. There seems to be a certain kind of intuitive understanding that becomes obvious after the first doses of a medication.

It is quite hard to explain, but it seems like the brain knows exactly what it needs. Unless you have uncontrollable addictive tendencies, you will automatically gravitate towards the dosage necessary to restore chemical balance. You will redose when it is needed, and you will notice quickly if a medication is not the right choice for you.

I was and still am very happy with the initial methylphenidate IR and Concerta (same compound but XR). I have tried different options over time, but they did not work as well or had side effects that I was not comfortable with. For example, I noticed that contrary to MPH, Adderall is destabilizing to my emotional state and thus was definitely not suitable for productivity, and the crash mainly consists of horrible mood swings and apathy, which is not the case for me on methylphenidate. On Vyvanse this was better, but the effects themselves still somewhat comparable to those of Adderall. I have not tried Strattera yet, but I do not really see a reason to these days.

On this note, I've talked with a lot of those affected by now, and choice and benefit of medications appears highly subjective. It seems like there is no way of knowing without trying it out.