FATE's Initiated by Players

So, there's a lot of in game controversy on how long the FATE's that need to be initiated by players stay up for before disappearing if they are left untouched.

I've seen a lot of people start the Svara FATE chain when half the populace is at The Eyes Have It FATE. Same goes for the Dark Devices chain while everyone is at Go, Go, Gorgimera. It tends to pretty much kill the interest in these two big boss FATE's and have people go for the chain FATE's, even if the bosses in question are at 30%-50% health left.

So can we get a Developer response or does anyone know how long these FATE's NPC's stay up for before they disappear? Do different trigger NPC's stay up for longer than others?