The only time I've run into the time limit is when facing multiple G-rank monsters, online and solo. I don't really think the time limit means anything, as it's either completely redundant in multiplayer where four players is easy mode, or a bit annoying as an auto-fail mechanic when you've been fighting for nearly an hour in single player. If you can survive that long against a series of monsters without carting three times, it feels a bit rubbish to have the time limit be what fails the quest rather than getting eaten.I could understand dropping the timer as it's not really a factor outside of the super massive gimmick monsters, but I cannot imagine a single monster in the game taking anyone 50 minutes on their own unless they're being willfully ignorant of how gear progression works and cannot grasp the basic gameplay fundamentals. I know I'm biased since I'm a series-long vet, but people that can't put away even a basic wyvern in 50 minutes should not be playing the series at all: it is so not for them.
I suppose it exists to stop players cheating their way through a tough quest by running back to camp to sleep each time they get hit, so it's encouraging 'learn to do it right' over trading hits.