Im not sure how long you must kill a boss on Path B either. But my longest was around 9 mins and i did not see any warning telling me of any timer going off soon. So I'm guessing it will be around 10-20mins
Bosses like the Red Dragon and Medusa can be killed without retreating, so there's no time limit. Time attacks are for stuff like Kraken, Killer Rabbit and the Chimera. I don't think there's a timer for the Gargoyle Gate? Nor for Cyclops... I believe there's a limit for the Wight, with a limited number of Goddess statues. Usually it dies by the first or second though.
Currently playing: SRW OE (PSP), Gundam Breaker (PS3)
There's a time limit for Gargoyle Gate. On Hard mode the gate destroyed two of the cannons that were deployed by human NPC allies, and then I smashed it up. I didn't get the talisman for defeating the gate.