So long as the mare is COMPLETELY OK, no problems no real temperature and is settled to nursing- which usually does take care of it anyway, I will leave her up to eight hours- longer if everything really is fine.
I would not give Oxytocin myself, even on Vet advice- BUT I do appreciate that some of you have to do this- not everyone has a Vet ten minutes away and not every Vet will tip out of their beds at one am for a small thing like a placenta- I do realise the constraints.
If I HAD to do it I would start off with half the suggested dosage- or work it out for myself using the mares weight and the leaflet that comes with the product.
Even then I would give less than the prescribed dose- this is potent stuff and you could end up with, worst case scenario- a prolapsed uterus.
Most times the placenta comes away on it's own, if not I will go in and have a root around BUT again, I have been doing this for a long time and was taught how to do this procedure by an Obstetrician, so it is not something to do if you are not sure of your self.
If you just go in and feel in the first instance, exactly as you would if the mare were foaling, yo often have all the info you need as you will be able to tell if the cervix is closing- in which case you do need help- it will not normally close down completely with a placenta still in there but in a very few cases it can happen.
If all is well and open and the placenta is just hanging by one horn- this is normal- then just leave it be, give her a nice fresh bucket of the coldest water you have and wait!!
I would observe closely and I would take temperates regularly but I wold not intervene, with no other reason, for eight hours- Yes, I would worry, but all my afterbirths have come away OK within that time frame- a mare aborting is another matter- as I would have the Vet to give antibiotics anyway I would also have them check the afterbirth and probably give her something to clear it- the couple of times I had this done they did not use Oxytocin there is something else on the market now that is less radical.
In both cases the mares did not even sweat and the placenta came within minutes.
I am glad yours worked out OK it is a very worrying situation.