How much seroquel (quetiapine) is considered to be a dangerous overdose?

Overdose = Dangerous. If the overdose was intentional then any dose is dangerous. A suicide attempt is a medical emergency. Seek assistance immediately. The manufacturer recommends a maximum dose of 800md/day. Overdoses any where from 9 grams to 30 grams have been reported. Usually not fatal, but fatalities have occurred. I would recommed emergency treatment with any overdose that alters one's mental status.
Any excess dosing. Don't do it. I could have told you even before the FDA picked up on it that this medicine would make the heart do flip-flops if people take extra tablets. The risk of cardiac rhythm problems is too great to exceed your prescribed dose, even just a little bit.