I have more issues with the respawn times.

The B ranks apparently already take an hour to respawn. So something dies in 10 seconds, you miss it, and have to wait another hour for your next chance. Basicly you have 4 minutes out of every day in which you can battle the monster, if you happen to be at the exact spot at the exact time, and manage to land a hit before anyone else finishes it off.

I've heard the A ranks are a day (silly, since a day didnt pass yet), and they speculate the S ranks take a week.