They dont take more than an armfull!
doctors advice is to avoid strenuous exertion for 24 hrs after giving blood. It doesn't take much more than that to regenerate the blood you have given.
If you do give blood, make sure you do not do so on an empty stomach (have a carb rich snack 60-90 mins before), and they will offer you a biscuit and a glass of juice afterwards- take it to keep your blood sugar level up.
Incidentaly, if you have had piercings or tattoo work done within the last 12 months, you will not be eligable for blood donation, due to the (slim) risk of blood born desease from such activities.
The same goes for if any of you are juicing intravenously, or taking any injectable recreational drug. Not that anyone here is thick enough to do that of course.