For me on Invega Sustenna it was almost immediately.
About 3 hours. 
For me it took several different APs . But with haldol it took 2 weeks.
I still hear voices.
The voices are not completely stopped by the drug that works the best for me. Since I was told that the voices would probably never leave I realized I had to find a way to get along with them. I have learned to do so the best I can at this point and stopped worrying about what they said indicated I was about and have learned to accept me better as well. Perhaps that’s the point.
Geodon took away my voices almost immediately, but then made my heart race so fast that I thought I was going to have a heart attack. I had to switch to abilify. I am pretty stable now.
Well it doesn’t completely remove the voices, but after 1 day I become noticeably less psychotic on cloziril. I go from being on a incoherent delusional level to a functional level.
After finally finding the right med. Then the right dose which took a few months, it was an overnight difference. Like a switch was flipped off in my head.
It took a while to find the right combo… but they started to decrease and change after a few months. I still hear them sometimes… but it’s usually with a panic attack or a stress spike.
Doesn’t outright stop them, but really helped the day I got on the right dose.