Boss regen rates

Okay you guys, prepare for something shocking. I was farming steel a few days ago, and I had a habbit of leaving MMs nearly dead in case some people, arrogant hermits or otherwise, wanted to just jump in. No one came in for the longest time here though.

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In the last SS, Heaven's Hammer recharged. That's just short of 10 minutes!

Also, during the entire coincidental experiment, I visited the ground floor occasionally to mop up the Iron Hogs for Steel Hooves, so inevitably I PGed MM occasionally. The boss life meter was pasted on my screen about... 75% of the time?

EDIT: Oh yeah, the original reason why I brought this up was because when I was on my fishing trips, I would often try to get HH+PG kills so I could SS the drops clearly. This fails occasionally though, even if HH does work as intended and the fish loses what HP you could see in its health bar, and during the first few failures, I thought that I probably timed it wrong, so I decided to PG the fish some more. And more. And more. It was clear that the fish wasn't dying, and in fact its life went up! Usually you just see no sliver of HP left on its health bar, which means it's literally within 30k damage of dying.