What an interesting question. And unfortunately, my answer is going to be more complicated than just a timeframe.
I have been treating my depression for many years. At several points, I felt that medication was working. Sometimes within a few days – simply starting treatment can get a placebo effect into play and if you can take that and run with it, you may keep it going long enough to get into actual treatment within 2–3–4 weeks.
But sometimes it wasn’t going to treat it properly. So we would start a new med. And the whole process would start again.
The 2–3 times that I landed on a mediation that *did* work, I wasn’t entirely aware that it was working until at a certain point, I thought, “Hey! Wait a sec! I seem to be back to a more normal state!” – it can be very hard to recognize. Consider it like hitting your thumb with a hammer. Right off, you know it hurts. And then you spend some minutes/hours with it hurting, and slowly it goes away.
3 days later, do you even consider how “ok” your thumb feels? Nope. It’s hard to be so in touch with … everything. But this is also why you should be following regular visits with your prescribing doctor – they will bring you back “into the moment” to try and see, with you, where you are at in terms of progress.