They were kind of specific that the safety net is an *increased* chance of legendaries from 2 hours, scaling up to a guarantee after somewhere around 6 hours of active play.Quote from Fizpez
It's playing time - not real world time. Apparently they also have some means of insuring its not just "log in and stand around in town" while you go to work or whatever. I'd say its probably in the neighborhood of a 2 hr max - assuming you're playing. It could be some combo of play time over monsters killed ratio.
I seriously doubt they will ever tell us the exact mechanics behind the "safety net" as there are plenty of people who will immediately go looking for a way to abuse it.
My advice? Open chests and break objects, as well as killing mobs. I've got a lot of legendaries from objects as well as monsters. Don't spend too much time on it, but bring some splash damage or damaging mobility skills and break some barrels, and take the extra seconds to click on a chest.