The medical literature is not our friend in this. When I was researching the chapter on 3rd stage for the new edition of Ob Myths (book is now 2/3 written), I found two studies looking at the relationship of 3rd stage duration and hemorrhage. Both studies found that it was not linear. There was a spike in hemorrhage incidence at about 30 min. The authors concluded that this was the safe limit for waiting. But if you think about it, you will see that extraction at whatever point in time would precipitate hemorrhage either as a consequence of the underlying reason why the placenta hadn't delivered, i.e. abnormally adherent placenta, or consequent to the procedure itself. The apparent threshold merely represents how long the typical practitioner was willing to wait before trying manual extraction. I think what does make sense, though, is don't try to get a stubborn placenta out at home. You want to be in an environment prepared to deal with it if all hell breaks loose along with the placenta.
-- Henci