Swallowing a bit of placenta is reportedly very effective in getting the uterus to start firming up. When women are left to their own devices, often they will without thinking about it gently rub their abdomens, which helps it to firm up. And of course nursing immediately, or even just having the baby up near the breasts, will stimulate oxytocin. No uterine massage should be done until after the placenta is out -- could cause part of it to detach prematurely. No cord traction, which could do the same thing.
It's hard to gauge what is normal blood loss because it's different for everybody. A general rule of thumb though is that if you are soaking two menstrual pads in an hour (not including the initial gush of blood that may come with the separation and birth of the placenta) you are bleeding too much and should seek help. A slower hemorrhage can often resolve itself, as the body expels blood clots over time.
I don't really know how likely it would be to have a fast hemorrhage given that the mother is healthy and the birth process undisturbed, but I would bet my life that it's quite rare. I thought this was interesting, also from the link I posted above:
"Botha (1968) attended over 26,000 Bantu women over 10 years, and reports that "a retained placenta was seldom seen…blood transfusion for postpartum haemorrhage was never necessary." Bantu women deliver both baby and placenta while squatting, and the cord is not attended to until the placenta delivers itself by gravity.
There is some evidence that the practice of clamping the cord, which is not practiced by indigenous cultures, contributes to both PPH and retained placenta by trapping extra blood (around 100ml, as described above) within the placenta. This increases placental bulk, which the uterus cannot contract efficiently against, and which is more difficult to expel. (Walsh 1968)
Other western practices that may contribute to PPH include the use of oxytocin for induction and augmentation (speeding up labour) (Brinsden 1978, McKenzie 1979), episiotomy or perineal trauma, forceps delivery, caesarean and previous caesarean (because of placental problems- see Hemminki 1996)."