I'd say around five days to honestly answer the question, i.e.
consecutive days without sleeping, obviously on speed.
But far more damaging/memorable is prolonged use where taking meth, eating, showering etc become just rostered activities in a schedule. I've had
month long benders where every three days or so you force feed yourself a banana and an orange, then lie down for a few hours without moving. It doesn't count as real sleep, as I recall constantly looking across to the clock- I had it all planned out: start work at 11am, so I'll wake up at 8am, have a hit, do laundry, eat some fruit, shower, then go to work. When you get into those routines where using (meth)amphetamines is the focus, and everything else you have to do revolves around 'when can I have a hit'?, working out priorities so 'I am not too fucked up at work, and am not too fucked at the end of the day so I can have a drink or some benzos/ H and just (maybe) pass out tonight', and then inevitably atfer work the first thing you do is score just so you'll have something to get you to work the next day.
I can operate surprisingly well in these states of mind. I get peripheral visuals, i.e. not hallucinations where shit is melting and morphing in front of me, but rather in the corner of my eye, I'll see something that just isn't real, or the room will start filling up with smoke whilst watching TV, or generally weird shit. Or driving... don't get me started about that! Concentrating so hard on driving, anything you're not looking directly at starts playing up.
My mantra was "I'm glad I'm totally fucked up on speed, or else I might have to do something about that...."eg. dinosaur at my window, giant rollerskating spider, tunnel in the floor by my feet. And when I would look directly at these things they'd turn out to be- a tree branch swaying in the wind, someone driving past with a bicycle on a bike rack on the back of their car, or a sock on the floor, respectively. Mentally I was working a pretty monotonous job which I could do quite easily- talking on the phone my voice would stutter a little bit, but I had done it so many times I could allow my mind to wander and still perform my duties. But the greatest fear was getting too excited around colleagues and not being able to stop talking on some strange tangent whilst on a smoko- the stress of being in the workplace was the worst part, where that level of drug use would be very much frowned upon.
It was a good lifestyle while it lasted, but I always had a way out- I had bought a ticket to go overseas, so I really let my hair down

If I didn't know I'd be leaving, I'd never have let shit get so bad. No regrets.