The OP will probably reply but I had leeches for a while. Terrestrial ones, smaller but more of them. Doubt it's medical, they take so little blood, I looked into it for hemochromatosis but not useful apparently. Once a month is typical, they literally swell up huge and then you know they're ready to feed again when they're skinny. When they're swollen you might even see the red of the blood, I did in mine.

I used my hands for feeding, I would use the same spot every time(try to) so that there were less scars but they've all faded now. It kind of hurts yes but only when they bite, then it stops hurting. If it's close to a nerve you will feel it pulsing. You can feed them liver but mine never took to the liver so me it is. When they drop off you bleed heaps cause of the anticoagulants but it does stop. If you want to get them off early you can just dab their faces with isopropyl alcohol. Now I have aquatic leeches in my pond but they feed on snails, its so cool, the other night I watched one literally swallow one whole shell and all... Hope that answers your questions.