For classical antidepressants Chemicals tend to function pretty fast but the effect is designed to be subtle because you need to live a normal life, capable of dealing with and feeling both ups and downs. not be perpetually smiling or brain dead.
After some time “a few weeks” of taking a daily dose your body and mind adjust to the subtle changes causing a stable therapeutic effect. At least thats what I understand is the idea.
But lately there is also sm of a psychedelic renaissance of medicine and they work entirely different where a single dose within a therapeutic setting creates a longer lasting feeling of increased well being.
Nasal Ketamine seems to receive allot of attention, near instantly improving the condition. They tend to need 1 dose every 5 weeks so its less addictive then classic medicine. But i do admit it instant improvement for treatment resistant medications is a bold claim and still subject of more research.