They can stay attached until full. They become fat and fall off. The amount of blood being sucked out depends on the size of the leech and when it last ate. Ask your local hospital ER staff, they may know
How Much Blood Can A Leech Suck Out Of A Persons Body?
10 something but, you lose more blood when you get a little cut on your finger. Also to get rid of them sprinkle salt on them.
One hit million I guess