Hey resident lawyer dudes! How much blood can we take before getting into trouble?!
Sadly, I don't think Truean has been on in some time. Plus they don't give legal advice over the internet.
I think the point that it becomes gross negligence is where the blood loss causes long-term harm.
Uh, I guess it depends on what you meaning by taking. You mean like, drawing blood from someone?
If so, then, as long as you have permission (and as long as you're a legal practicing physician, which may be a legal requirement in some places), it likely depends on the amount of blood one can draw from a person at any given time, and the maximum amount one can draw cumulatively over a certain period. These are techniqual guidelines, not legal ones (unless your particular legislation has covered that as well), btw, and vary a lot between patients and depends a lot on the context of the situation.
Speaking plainly, Akura is correct. If you draw so much blood that you're actualy endangering the patient, and there's proof that your act of drawing blood was done in a context in which it was inappropriate to do so and/or it was in a large enough amount that the patient would likely suffer serious (long term or not) consequences from it, then you can likely fall under a malpractice suit or somesuch, unless you drew blood in a non medical context, in which case I'd call the local vampire hunter.