As far as I know is in the entire fight, which usually don't last very long.
But.... I've also heard that you are only considered as participant, if you dealt damage to the boss in the last 15 minutes. For an example, Gaz'Haragoth respawns, you throw 1 SD and leave, and then he dies 3 hours later, you wouldn't get any loot. (this one seems to be true, because I'm pretty sure it happened to me already)
But I know for sure it doesn't work for mini-falcon bosses, because I hunt in Falcon Castle, so sometimes I'll deal some damageto Preceptor Lazare, but won't kill, since it lowers exp/h, and then hours later I'll receive a message about his loot.
So, maybe each boss has a different mechanic conserning time. But that usually isn't a big problem, since most bosses will die in a few minutes. Imo the only exception is Gaz'Haragoth and Omrafir