Dori, do not have more than one bag at once, it is dangerous, a few hours extra is better than risking your life.
In my hospital the fastest they will give a unit is 1.30 hours but only if theres an urgency or something. THe general level is 2 hours per bag but only if you are young fit and healthy. Sometimes they give me over 3 hours.
I find that i react more when blood is given fast even two hours, i will get a temperature by the 2nd bag. TOoday i had tt over 3 hours and got no reaction at all-1st time in years. so the slower the better i say. Obviously the transfusion cant be longer than 4 hours, generally i would say 2-3 hours is the best.