1. "Therapeutic" just means a high enough dosage to have an effect. Many psych medications stay in your bloodstream for a long time, though not at full strength of course. I tried to google the "half-life" of seroquel, but couldn't find it. You should have received a pamphlet with your med, there is a lot you should be looking out for, like drinking a full glass of water when you take the pill, etc.

2. & 3. From what I saw on the google, yes it is common to take it at night.