I’m an athletic trainer. Athletic training is sports medicine, not personal training for those who aren’t aware of the profession. I have a lot of training and experience with proper hydration.
Dehydration and the other stuff that goes along with it gets plenty of press when things go wrong. Honestly, human bodies are far more resilient than we give them credit for. Quite often with the cases of dehydration/heat related deaths, there were other factors at play. The heat and water definitely exacerbated it though.
Training intensity, duration, heat index (heat and humidity), physical condition, and pre-activity hydration all must be taken into account. But if you’re well hydrated before training, are in average shape, don’t have any underlying health issues, aren’t taking anything that dehydrates you (like some prescription meds), and you’re going for an hour and a half at a moderate pace; you shouldn’t have any problems. You should obviously listen to your body and be conscientious of symptoms - Dizziness, shaking, etc.
I train for an hour and a half. Sometimes it’s pretty hot in there, especially during the summer. I sweat far more than anyone else in the dojo. My gi is always soaked afterwards. For reference, my wife picked up my gi one time after I came home from class one time and asked if it was ready to go in the dryer - by how wet it was, she thought I just pulled it out of the washer. And I’ve never had to stop for a drink. Never felt any symptoms of dehydration.
For the record, we’re allowed to bring water. We can keep a bottle right on the floor out of the way. No ones ever been told they can’t drink. Hardly anyone ever has taken a drink.
Moral of the story, push yourself, but don’t be stupid. Listen to your body.
Furthermore, many experts say if you’re thirsty, you’ve been dehydrated for longer than you think. And water doesn’t absorb instantly.
I used to weigh athletes in and out of practice. They’d come into the training room and step on the scale. After practice, they’d get changed into dry and similar clothing, and weigh themselves again. Then they’d be responsible to drink the amount of water they lost during practice. I’ve had to deal with heat/dehydration issues during my career. Especially during fall pre-season - football, soccer, etc. There were a fair number of times I had to pull someone off the field and get them into a cool place and get them drinking, but if you’re looking at the number of people practicing and all the other stats, it wasn’t much at all from a numbers standpoint. And most often, they didn’t drink AND eat correctly beforehand. People tend to forget about the eating part of the equation.
The point, I guess, I’d be smart. But don’t think that if you don’t drink as soon as you get a little thirsty of hot that you’re going to die