From my experience, Seroquel has completely halted manic episodes within 3 to 4 days at very low dosages. Unfortunately, it makes me feel very lethargic and really slow probably because I've never taken it long term.
A friend of mine took 300 mg/day awhile ago, which they upped to 600 mg after he couldn't fall asleep with it anymore. Eventually, he tried to OD on it and took the rest of his supply. Apparently, the LD50 must be very high as he only slept for 15 hours and woke up extremely uncoordinated and groggy.
Seroquel appears to be very safe... I would keep alittle around for an emergency mania med but consult with your doc first to find out about any serious interactions...