The Blood Donation Machine allows you to trade half a heart pre-Womb or a whole heart in or after the Womb for some amount of money. Additionally, there is a chance that the machine will explode and give you a Blood Bag, which gives you another heart and heals you five hearts (unless you're playing as ???).
I've read that the average number of usages to obtain a Blood Bag is around eight. I've never made it as high as eight personally, but it's supposedly possible to go well above that.
Is there a maximum or minimum number of times before a Blood Bag will drop? If I have a certain number of hearts, am I guaranteed a Blood Bag? Can it drop on the first use of the machine?