Are you discontinuing Seroquel, or wondering how long it takes for a single 100mg dose to clear your system?
The half-life of Seroquel is about 6 hours. This means that in 6 hours, half of the dose is gone. In another 6 hours, half the remaining dose is gone, so your serum level is about down to 25. In 24 hours, you are down to about 6mg, which is probably negligible, and in 30 hours or so it will have cleared your system.
If you're discontinuing the drug, that's a little different. Read my SharePost: Going Off a Medication? It May Not Be Easy.