However long it takes for the pill to kick in, at a moderate dose. It's a direct effect of the drug, not a long-term brain damage sorta thing (well, AFAIK with most benzos), so goes away after withdrawal. You may not realise it at the time, just like drunk people don't realise how many regrets they're gonna have in the morning.
I found it to be clearly dose-related, such that 0.5mg Kpin produces only minimal impairment of intellect, while 2-3mg makes me dumb as bricks. I mean, not to the point where I'm a babbling idiot, but enough to prevent me making progress with my education and enough to make me say the stupidest things to people. Like being drunk but without the slurring, stumbling around bit. Y'know, misinterpreting cues, speaking your mind where inappropriate, etc. I can always
tell if someone I'm speaking to is on benzos, even if I don't know them and even at moderate/low doses.
I'm so anti-benzo these days because I know there are drugs that work for mood disorders yet don't screw up intelligence. Serotonin and dopamine-acting drugs instantly come to mind, with the addition of beta-blockers for anxiety, for example. If I were you I'd be taking carvedilol with Adderall, not Kpin.